Update WString.cpp

realloc() is called with newSize > 0 (at least 16), so newbuffer==0 means the old memory was not deallocated. Therefore, the pointer should still point to the old buffer. This change should resolve issue esp8266#3516.

cores/esp8266/WString.cpp

@@ -159,7 +159,6 @@ unsigned char String::changeBuffer(unsigned int maxStrLen) {
         buffer = newbuffer;
         return 1;
     }
-    buffer = newbuffer;
     return 0;
 }