Slight improvement to the line parsing logic, saving 8 cycles per character

Marlin/Marlin_main.cpp

@@ -1094,9 +1094,10 @@ inline void get_serial_commands() {
   /**
    * Loop while serial characters are incoming and the queue is not full
    */
-  while (commands_in_queue < BUFSIZE && MYSERIAL.available() > 0) {
+  int c;
+  while (commands_in_queue < BUFSIZE && (c = MYSERIAL.read()) >= 0) {
 
-    char serial_char = MYSERIAL.read();
+	char serial_char = c;
 
     /**
      * If the character ends the line
@@ -1196,9 +1197,9 @@ inline void get_serial_commands() {
       // The command will be injected when EOL is reached
     }
     else if (serial_char == '\\') {  // Handle escapes
-      if (MYSERIAL.available() > 0) {
+      if ((c = MYSERIAL.read()) >= 0) {
         // if we have one more character, copy it over
-        serial_char = MYSERIAL.read();
+        serial_char = c;
         if (!serial_comment_mode) serial_line_buffer[serial_count++] = serial_char;
       }
       // otherwise do nothing