Avoid crash after function syntax error in a replication worker.

If a syntax error occurred in a SQL-language or PL/pgSQL-language
CREATE FUNCTION or DO command executed in a logical replication worker,
we'd suffer a null pointer dereference or assertion failure.  That
seems like a rather contrived case, but nonetheless worth fixing.

The cause is that function_parse_error_transpose assumes it must be
executing within the context of a Portal, but logical/worker.c
doesn't create a Portal since it's not running the standard executor.
We can just back off the hard Assert check and make it fail gracefully
if there's not an ActivePortal.  (I have a feeling that the aggressive
check here was my fault originally, probably because I wasn't sure if
the case would always hold and wanted to find out.  Well, now we know.)

The hazard seems to exist in all branches that have logical replication,
so back-patch to v10.

Maxim Orlov, Anton Melnikov, Masahiko Sawada, Tom Lane

Discussion: https://postgr.es/m/b570c367-ba38-95f3-f62d-5f59b9808226@inbox.ru
Discussion: https://postgr.es/m/adf0452f-8c6b-7def-d35e-ab516c80088e@inbox.ru

src/backend/catalog/pg_proc.c

@@ -1012,7 +1012,6 @@ function_parse_error_transpose(const char *prosrc)
 {
 	int			origerrposition;
 	int			newerrposition;
-	const char *queryText;
 
 	/*
 	 * Nothing to do unless we are dealing with a syntax error that has a
@@ -1030,11 +1029,22 @@ function_parse_error_transpose(const char *prosrc)
 	}
 
 	/* We can get the original query text from the active portal (hack...) */
-	Assert(ActivePortal && ActivePortal->status == PORTAL_ACTIVE);
-	queryText = ActivePortal->sourceText;
+	if (ActivePortal && ActivePortal->status == PORTAL_ACTIVE)
+	{
+		const char *queryText = ActivePortal->sourceText;
 
-	/* Try to locate the prosrc in the original text */
-	newerrposition = match_prosrc_to_query(prosrc, queryText, origerrposition);
+		/* Try to locate the prosrc in the original text */
+		newerrposition = match_prosrc_to_query(prosrc, queryText,
+											   origerrposition);
+	}
+	else
+	{
+		/*
+		 * Quietly give up if no ActivePortal.  This is an unusual situation
+		 * but it can happen in, e.g., logical replication workers.
+		 */
+		newerrposition = -1;
+	}
 
 	if (newerrposition > 0)
 	{