fix(weblog): guard against a weblog_buffer of 0 (pop_front on empty deque)

WebLogService::operator<< trims with `log_messages_.size() >= limit_log_messages_`.
size() is unsigned, so with limit_log_messages_ == 0 the condition holds even for an
empty deque and pop_front() runs on it - undefined behaviour, in practice heap
corruption. On a PSRAM board the stored value survives a reboot, so the device crashes
again as soon as the first log message is written.

Nothing clamps the value on the way in: start() copies settings.weblog_buffer with no
lower bound, the PSRAM branch of operator<< passes it straight into
limit_log_messages_, and ArduinoJson's | in WebSettings::update() only substitutes the
default for a missing or type-incompatible key - a stored 0 is a valid uint16_t.

- operator<<: skip the trim loop on an empty deque
- start(): keep the existing default when the stored value is 0
- getSetValues(): a missing or zero max_messages keeps the current size instead of
  storing 0
- WebSettings::update(): repair a stored 0, and derive the psram/no-psram default once
  instead of twice

Refs #3237

src/web/WebLogService.cpp

@@ -44,10 +44,13 @@ void WebLogService::begin() {
 // apply the user settings
 void WebLogService::start() {
     EMSESP::webSettingsService.read([&](WebSettings & settings) {
-        maximum_log_messages_ = settings.weblog_buffer;
-        limit_log_messages_   = maximum_log_messages_;
-        compact_              = settings.weblog_compact;
-        level_                = (uuid::log::Level)settings.weblog_level;
+        // a stored value of 0 would make limit_log_messages_ 0, see the guard in operator<<
+        if (settings.weblog_buffer) {
+            maximum_log_messages_ = settings.weblog_buffer;
+        }
+        limit_log_messages_ = maximum_log_messages_;
+        compact_            = settings.weblog_compact;
+        level_              = (uuid::log::Level)settings.weblog_level;
     });
     uuid::log::Logger::register_handler(this, level_);
     if (level_ == uuid::log::Level::OFF) {
@@ -80,7 +83,9 @@ void WebLogService::operator<<(std::shared_ptr<uuid::log::Message> message) {
         }
     }
 #endif
-    while (log_messages_.size() >= limit_log_messages_) {
+    // size() is unsigned, so with limit_log_messages_ == 0 this condition is always true
+    // and pop_front() would run on an empty deque (undefined behaviour)
+    while (!log_messages_.empty() && log_messages_.size() >= limit_log_messages_) {
         log_messages_.pop_front();
     }
 
@@ -210,9 +215,14 @@ void WebLogService::getSetValues(AsyncWebServerRequest * request, JsonVariant js
     }
 
     // POST - write the settings
-    level_                = json["level"];
-    maximum_log_messages_ = json["max_messages"];
-    compact_              = json["compact"];
+    level_ = json["level"];
+    // keep the current size if the key is missing or 0 - a 0 would be stored and make
+    // limit_log_messages_ 0 on the next boot
+    const size_t max_messages = json["max_messages"] | maximum_log_messages_;
+    if (max_messages) {
+        maximum_log_messages_ = max_messages;
+    }
+    compact_ = json["compact"];
 
     uuid::log::Logger::register_handler(this, level_);
     if (level_ == uuid::log::Level::OFF) {

src/web/WebSettingsService.cpp

@@ -328,10 +328,12 @@ StateUpdateResult WebSettings::update(JsonObject root, WebSettings & settings) {
     settings.email_subject  = root["email_subject"] | FACTORY_EMAIL_SUBJECT;
 
     // if no psram limit weblog buffer to 25 messages
-    if (EMSESP::system_.PSram() > 0) {
-        settings.weblog_buffer = root["weblog_buffer"] | EMSESP_DEFAULT_WEBLOG_BUFFER;
-    } else {
-        settings.weblog_buffer = root["weblog_buffer"] | 25; // limit to 25 messages if no psram
+    const uint16_t weblog_buffer_default = (EMSESP::system_.PSram() > 0) ? EMSESP_DEFAULT_WEBLOG_BUFFER : 25;
+    settings.weblog_buffer               = root["weblog_buffer"] | weblog_buffer_default;
+    // a stored 0 is a valid uint16 and passes the | default above, but would make
+    // limit_log_messages_ 0 in WebLogService
+    if (settings.weblog_buffer == 0) {
+        settings.weblog_buffer = weblog_buffer_default;
     }
 
     // save the settings if changed from the webUI