Add check to avoid buffer overflow on progmem2char

transfer is limited to 300 char string, if over, string is ignored

esp8266/webinterface.cpp

@@ -332,7 +332,8 @@ const char POLLING_ID[]PROGMEM ="POLLING";
 char * progmem2char(const char* src)
 {
   static char buffer[300];
-  strcpy_P(buffer,src);
+  buffer[0]=0;
+  if(strlen_P(src)<299)strcpy_P(buffer,src);
   return buffer;
 }