Don't leak malloc'd error string in libpqrcv_check_conninfo().

We leaked the error report from PQconninfoParse, when there was
one.  It seems unlikely that real usage patterns would repeat
the failure often enough to create serious bloat, but let's
back-patch anyway to keep the code similar in all branches.

Found via valgrind testing.
Back-patch to v10 where this code was added.

Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -246,9 +246,15 @@ libpqrcv_check_conninfo(const char *conninfo)
 
 	opts = PQconninfoParse(conninfo, &err);
 	if (opts == NULL)
+	{
+		/* The error string is malloc'd, so we must free it explicitly */
+		char	   *errcopy = err ? pstrdup(err) : "out of memory";
+
+		PQfreemem(err);
 		ereport(ERROR,
 				(errcode(ERRCODE_SYNTAX_ERROR),
-				 errmsg("invalid connection string syntax: %s", err)));
+				 errmsg("invalid connection string syntax: %s", errcopy)));
+	}
 
 	PQconninfoFree(opts);
 }