Suppress uninitialized-variable warning in guc.c.

Back-patch 26812bcaa into 9.5, to suppress one more warning
in out-of-support branches.  (Doesn't appear to be needed
before 9.5.)

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

src/backend/utils/misc/guc.c

@@ -9124,6 +9124,8 @@ RestoreGUCState(void *gucstate)
 		if (varsourcefile[0])
 			read_gucstate_binary(&srcptr, srcend,
 								 &varsourceline, sizeof(varsourceline));
+		else
+			varsourceline = 0;
 		read_gucstate_binary(&srcptr, srcend,
 							 &varsource, sizeof(varsource));
 		read_gucstate_binary(&srcptr, srcend,