Ignore not-yet-defined Portals in pg_cursors view.

pg_cursor() supposed that any Portal it finds in the hash table must
have sourceText set up, but there's an edge case where that is not so.
A newly-created Portal has sourceText = NULL, and that doesn't change
until PortalDefineQuery is called.  In SPI_cursor_open_internal,
we perform GetCachedPlan between CreatePortal and PortalDefineQuery,
and it's possible for user-defined code to execute during that
planning and cause a fetch from the pg_cursors view, resulting in a
null-pointer-dereference crash.  (It looks like the same could happen
in exec_bind_message, but I've not tried to provoke a failure there.)

I considered trying to fix this by setting sourceText sooner, but
there may be instances of this same calling pattern in extensions,
and we couldn't be sure they'd get the memo promptly.  It seems
better to redefine pg_cursor as not showing Portals that have
not yet had PortalDefineQuery called on them, which we can do by
just skipping them if sourceText is still NULL.

(Before a1c692358, pg_cursor would instead return a row with NULL
in the statement column.  We could revert to that behavior but it
doesn't really seem like a better definition, especially since our
documentation doesn't suggest that the column could be NULL.)

Per report from PetSerAl.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/CAKygsHTBXLXjwV43kpZa+Cs+XTiaeeJiZdL4cPBm9f4MTdw7wg@mail.gmail.com

src/backend/utils/mmgr/portalmem.c

@@ -1150,6 +1150,9 @@ pg_cursor(PG_FUNCTION_ARGS)
 		/* report only "visible" entries */
 		if (!portal->visible)
 			continue;
+		/* also ignore it if PortalDefineQuery hasn't been called yet */
+		if (!portal->sourceText)
+			continue;
 
 		values[0] = CStringGetTextDatum(portal->name);
 		values[1] = CStringGetTextDatum(portal->sourceText);