Fix is_M29

Followup for #13013

Marlin/src/gcode/queue.cpp

@@ -526,8 +526,9 @@ void gcode_line_error(PGM_P const err, const int8_t port) {
 
 #endif // BINARY_FILE_TRANSFER
 
-FORCE_INLINE bool is_M29(const char * const cmd) {
-  return cmd[0] == 'M' && cmd[1] == '2' && cmd[2] == '9' && !WITHIN(cmd[3], '0', '9');
+FORCE_INLINE bool is_M29(const char * const cmd) {  // matches "M29" & "M29 ", but not "M290", etc
+  const char * const m29 = strstr_P(cmd, PSTR("M29"));
+  return m29 && !NUMERIC(m29[3]);
 }
 
 /**