Fix InitializeSessionUserId not to deference NULL rolename pointer.

Dmitriy Sarafannikov, reviewed by Michael Paquier and Haribabu Kommi,
with a minor fix by me.

src/backend/utils/init/miscinit.c

@@ -474,6 +474,7 @@ InitializeSessionUserId(const char *rolename, Oid roleid)
 {
 	HeapTuple	roleTup;
 	Form_pg_authid rform;
+	char 	*rname;
 
 	/*
 	 * Don't do scans if we're bootstrapping, none of the system catalogs
@@ -485,16 +486,25 @@ InitializeSessionUserId(const char *rolename, Oid roleid)
 	AssertState(!OidIsValid(AuthenticatedUserId));
 
 	if (rolename != NULL)
+	{
 		roleTup = SearchSysCache1(AUTHNAME, PointerGetDatum(rolename));
+		if (!HeapTupleIsValid(roleTup))
+			ereport(FATAL,
+					(errcode(ERRCODE_INVALID_AUTHORIZATION_SPECIFICATION),
+					 errmsg("role \"%s\" does not exist", rolename)));
+	}
 	else
+	{
 		roleTup = SearchSysCache1(AUTHOID, ObjectIdGetDatum(roleid));
 		if (!HeapTupleIsValid(roleTup))
 			ereport(FATAL,
 					(errcode(ERRCODE_INVALID_AUTHORIZATION_SPECIFICATION),
-				 errmsg("role \"%s\" does not exist", rolename)));
+					 errmsg("role with OID %u does not exist", roleid)));
+	}
 
 	rform = (Form_pg_authid) GETSTRUCT(roleTup);
 	roleid = HeapTupleGetOid(roleTup);
+	rname = NameStr(rform->rolname);
 
 	AuthenticatedUserId = roleid;
 	AuthenticatedUserIsSuperuser = rform->rolsuper;
@@ -520,7 +530,7 @@ InitializeSessionUserId(const char *rolename, Oid roleid)
 			ereport(FATAL,
 					(errcode(ERRCODE_INVALID_AUTHORIZATION_SPECIFICATION),
 					 errmsg("role \"%s\" is not permitted to log in",
-							rolename)));
+							rname)));
 
 		/*
 		 * Check connection limit for this role.
@@ -538,11 +548,11 @@ InitializeSessionUserId(const char *rolename, Oid roleid)
 			ereport(FATAL,
 					(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
 					 errmsg("too many connections for role \"%s\"",
-							rolename)));
+							rname)));
 	}
 
 	/* Record username and superuser status as GUC settings too */
-	SetConfigOption("session_authorization", rolename,
+	SetConfigOption("session_authorization", rname,
 					PGC_BACKEND, PGC_S_OVERRIDE);
 	SetConfigOption("is_superuser",
 					AuthenticatedUserIsSuperuser ? "on" : "off",