Fix for MD5 leak bug, issue #7195 (#7197)

Co-authored-by: Luiss <luiss@mind.cc>
Co-authored-by: david gauchard <gauchard@laas.fr>
Co-authored-by: Develo <deveyes@gmail.com>
Co-authored-by: Earle F. Philhower, III <earlephilhower@yahoo.com>

cores/esp8266/MD5Builder.cpp

@@ -55,6 +55,7 @@ bool MD5Builder::addStream(Stream & stream, const size_t maxLen){
         // read data and check if we got something
         int numBytesRead = stream.readBytes(buf, readBytes);
         if(numBytesRead< 1) {
+            free(buf); // release the buffer
             return false;
         }