fix possible free of null in String

[Links2004]

No description provided.

[igrr]

The free() function deallocates the memory allocation pointed to by ptr. If ptr is a NULL pointer, no
 operation is performed.

[Links2004]

it will generate an error message when setDebugOutput is active.

[pgollor]

Sorry for the question. But why we are using malloc and free and not new and delete. Malloc is for c and new is the better way in CPP to allocate memory.
Or am i wrong?

[Links2004]

and new and delete are implemented with malloc and free ;)
https://github.com/esp8266/Arduino/blob/esp8266/hardware/esp8266com/esp8266/cores/esp8266/abi.cpp#L28

[pgollor]

thanks @Links2004 :)

[mangelajo]

+1, let's avoid unnecessary error messages in debug if possible.