Plane: send_text_P bug fix for short messages

ArduPlane/GCS_Mavlink.pde

@@ -1040,6 +1040,9 @@ GCS_MAVLINK::send_text_P(gcs_severity severity, const prog_char_t *str)
     uint8_t i;
     for (i=0; i<sizeof(m.text); i++) {
         m.text[i] = pgm_read_byte((const prog_char *)(str++));
+        if (m.text[i] == '\0') {
+            break;
+        }
     }
     if (i < sizeof(m.text)) m.text[i] = 0;
     mavlink_send_text(chan, severity, (const char *)m.text);