plugins/check_real.c - recv string null terminate

Recv into buffer is not properly null terminated prior to strstr and possible other string functions expecting a null termination. Simply take bytes received and use as an index to append \0 after. We are creating buffer[] with size of MAX_INPUT_BUFFER and recv with MAX_INPUT_BUFFER-1 so this should never overflow.
Merge branch 'cov-66525' into coverity