fix px4_log.c: avoid potential buffer overflow

The buffer length given to snprintf() is unsigned, so we must check for
an overflow after each call.

This happend for very long printf's (module documentation)

src/platforms/common/px4_log.c

@@ -106,22 +106,39 @@ __EXPORT void px4_log_modulename(int level, const char *moduleName, const char *
 
 			if (is_atty) { pos += snprintf(buffer + pos, max_length - pos, "%s", __px4_log_level_color[level]); }
 
+			if (pos >= max_length) { return; }
+
 			pos += snprintf(buffer + pos, max_length - pos, __px4__log_level_fmt __px4__log_level_arg(level));
 
+			if (pos >= max_length) { return; }
+
 			if (is_atty) { pos += snprintf(buffer + pos, max_length - pos, "%s", PX4_ANSI_COLOR_GRAY); }
 
+			if (pos >= max_length) { return; }
+
 			pos += snprintf(buffer + pos, max_length - pos, __px4__log_modulename_pfmt, moduleName);
 			va_list argptr;
 
+			if (pos >= max_length) { return; }
+
 			if (is_atty) { pos += snprintf(buffer + pos, max_length - pos, "%s", __px4_log_level_color[level]); }
 
+			if (pos >= max_length) { return; }
+
 			va_start(argptr, fmt);
 			pos += vsnprintf(buffer + pos, max_length - pos, fmt, argptr);
+
+			if (pos >= max_length) { return; }
+
 			va_end(argptr);
 			pos += snprintf(buffer + pos, max_length - pos, "\n");
 
+			if (pos >= max_length) { return; }
+
 			if (is_atty) { pos += snprintf(buffer + pos, max_length - pos, "%s", PX4_ANSI_COLOR_RESET); }
 
+			if (pos >= max_length) { return; }
+
 			// +1 for the terminating 0 char.
 			send_stdout_pipe_buffer(pos + 1);
 		}
@@ -197,12 +214,18 @@ __EXPORT void px4_log_raw(int level, const char *fmt, ...)
 
 			if (is_atty) { pos += snprintf(buffer + pos, max_length - pos, "%s", __px4_log_level_color[level]); }
 
+			if (pos >= max_length) { return; }
+
 			va_start(argptr, fmt);
 			pos += vsnprintf(buffer + pos, max_length - pos, fmt, argptr);
 			va_end(argptr);
 
+			if (pos >= max_length) { return; }
+
 			if (is_atty) { pos += snprintf(buffer + pos, max_length - pos, "%s", PX4_ANSI_COLOR_RESET); }
 
+			if (pos >= max_length) { return; }
+
 			// +1 for the terminating 0 char.
 			send_stdout_pipe_buffer(pos + 1);
 		}