git-remote-mediawiki: be more defensive when requests fail

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/mw-to-git/git-remote-mediawiki

@@ -912,6 +912,14 @@ sub mw_import_revids {
 
 		my $result = $mediawiki->api($query);
 
+		if (!$result) {
+			die "Failed to retrieve modified page for revision $pagerevid";
+		}
+
+		if (!defined($result->{query}->{pages})) {
+			die "Invalid revision $pagerevid.";
+		}
+
 		my @result_pages = values(%{$result->{query}->{pages}});
 		my $result_page = $result_pages[0];
 		my $rev = $result_pages[0]->{revisions}->[0];