Possible memory leak in MD5Builder

[lrodorigo]

Hi all,
I noticed a possible memory leak in MD5 Builder addStream member function.

Arduino/cores/esp8266/MD5Builder.cpp

Line 58 in eea9999

return false;

As you can see, if the readBytes fails, the function returns without freeing the allocated buffer.

[earlephilhower]

@lrodorigo Looks like a potential leak to me! Want to do a PR for it and add the free before the return false?