PR77765 exposed an ICE triggered in gfortran's acc routine parser by an
uninitialized proc_name. That situation occurred because the function
containing the acc routine directive has an error, so
gfc_current_ns->proc_name was never set.

Although it could be argued that the acc routine parser should not run
if any errors have been detected inside the routine containing such a
directive, this patch just teaches gfc_match_oacc_routine to check for
the existence of gfc_current_ns->proc_name before comparing the
procedure's name with the routine name specified by the user.

Is this patch OK for trunk and gcc7?

Thanks,
Cesar