FreeSWITCH-FS-7269

fix buffer length calculation in call to generate_m

This error was causing us to call generate_m with a buffer length that
was less than the strlen of the buffer we were passing.  The result
was truncated local SDP which would cause sofia to fail if the
truncation was in a particularly bad place.

Thanks to Anthony Minessale for the fix and working with me to
diagnose the issue.

FS-4293 --resolve