G4 shows status message only if no message is set already

- Address issue raised in #1961

Marlin/Marlin_main.cpp

 inline void gcode_G4() {
   millis_t codenum = 0;
 
-  LCD_MESSAGEPGM(MSG_DWELL);
-
   if (code_seen('P')) codenum = code_value_long(); // milliseconds to wait
   if (code_seen('S')) codenum = code_value_long() * 1000; // seconds to wait
 
   st_synchronize();
   refresh_cmd_timeout();
   codenum += previous_cmd_ms;  // keep track of when we started waiting
+
+  if (!lcd_hasstatus()) LCD_MESSAGEPGM(MSG_DWELL);
+
   while (millis() < codenum) {
     manage_heater();
     manage_inactivity();

Marlin/ultralcd.cpp

   s[i] = 0;
 }
 
+bool lcd_hasstatus() { return (lcd_status_message[0] != '\0'); }
+
 void lcd_setstatus(const char* message, bool persist) {
   if (lcd_status_message_level > 0) return;
   strncpy(lcd_status_message, message, 3*LCD_WIDTH);

Marlin/ultralcd.h

   int lcd_strlen_P(const char *s);
   void lcd_update();
   void lcd_init();
+  bool lcd_hasstatus();
   void lcd_setstatus(const char* message, const bool persist=false);
   void lcd_setstatuspgm(const char* message, const uint8_t level=0);
   void lcd_setalertstatuspgm(const char* message);