Reserve the [Exit] slot when splitting canned messages (#11095)

splitConfiguredMessages capped the split loop at
CANNED_MESSAGE_MODULE_MESSAGE_MAX_COUNT and then appended [Exit]
unconditionally, so messages[] could be written one past its last index
and messagesCount could exceed the array size.

Author: caveman99

src/modules/CannedMessageModule.cpp

@@ -179,7 +179,7 @@ int CannedMessageModule::splitConfiguredMessages()
     while (i < upTo) {
         if (this->messageBuffer[i] == '|') {
             this->messageBuffer[i] = '\0'; // End previous message
-            if (tempCount >= CANNED_MESSAGE_MODULE_MESSAGE_MAX_COUNT)
+            if (tempCount >= CANNED_MESSAGE_MODULE_MESSAGE_MAX_COUNT - 1)
                 break;
             tempMessages[tempCount++] = (this->messageBuffer + i + 1);
         }