Fix corner-case coredump in _SPI_error_callback().

I noticed that _SPI_execute_plan initially sets spierrcontext.arg = NULL,
and only fills it in some time later.  If an error were to happen in
between, _SPI_error_callback would try to dereference the null pointer.
This is unlikely --- there's not much between those points except
push-snapshot calls --- but it's clearly not impossible.  Tweak the
callback to do nothing if the pointer isn't set yet.

It's been like this for awhile, so back-patch to all supported branches.

src/backend/executor/spi.c

@@ -2419,6 +2419,9 @@ _SPI_error_callback(void *arg)
 	const char *query = (const char *) arg;
 	int			syntaxerrposition;
 
+	if (query == NULL)			/* in case arg wasn't set yet */
+		return;
+
 	/*
 	 * If there is a syntax error position, convert to internal syntax error;
 	 * otherwise treat the query as an item of context stack