Fix function return type confusion

When parse_hba_line's return type was changed from bool to a pointer,
the MANDATORY_AUTH_ARG macro wasn't adjusted.

This back-patches commit c74d586d2 into out-of-support branches,
pursuant to newly-established project policy.  The point is to
suppress scary-looking warnings so that people building these
branches needn't expend brain cells verifying that it's safe
to ignore the warnings.

Discussion: https://postgr.es/m/d0316012-ece7-7b7e-2d36-9c38cb77cb3b@enterprisedb.com

src/backend/libpq/hba.c

@@ -745,7 +745,7 @@ check_same_host_or_net(SockAddr *raddr, IPCompareMethod method)
 						authname, argname), \
 				 errcontext("line %d of configuration file \"%s\"", \
 						line_num, HbaFileName))); \
-		return false; \
+		return NULL; \
 	} \
 } while (0);