clipboard: don't crash if the source client does not send a mime type

Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/src/clipboard.c b/src/clipboard.c
index c1091a9..994c301 100644
--- a/src/clipboard.c
+++ b/src/clipboard.c
@@ -254,7 +254,7 @@
 
 	mime_types = source->mime_types.data;
 
-	if (pipe2(p, O_CLOEXEC) == -1)
+	if (!mime_types || pipe2(p, O_CLOEXEC) == -1)
 		return;
 
 	source->send(source, mime_types[0], p[1]);