libobs: Fix pasting filters crash when missing sources

Solves crash when a user tries to paste filters without selecting a
source or when pasting filters from a deleted source. This commit
checks if both sources are still valid before pasting.

This addresses Mantis Issue 1220
(https://obsproject.com/mantis/view.php?id=1220).

libobs/obs-source.c

@@ -427,6 +427,11 @@ static void duplicate_filters(obs_source_t *dst, obs_source_t *src,
 
 void obs_source_copy_filters(obs_source_t *dst, obs_source_t *src)
 {
+	if (!obs_source_valid(dst, "obs_source_copy_filters"))
+		return;
+	if (!obs_source_valid(src, "obs_source_copy_filters"))
+		return;
+
 	duplicate_filters(dst, src, dst->context.private);
 }