liblzma: Add dest and src NULL checks to lzma_index_cat.

The documentation states LZMA_PROG_ERROR can be returned from
lzma_index_cat. Previously, lzma_index_cat could not return
LZMA_PROG_ERROR. Now, the validation is similar to
lzma_index_append, which does a NULL check on the index
parameter.

src/liblzma/common/index.c

@@ -771,6 +771,9 @@ extern LZMA_API(lzma_ret)
 lzma_index_cat(lzma_index *restrict dest, lzma_index *restrict src,
 		const lzma_allocator *allocator)
 {
+	if (dest == NULL || src == NULL)
+		return LZMA_PROG_ERROR;
+
 	const lzma_vli dest_file_size = lzma_index_file_size(dest);
 
 	// Check that we don't exceed the file size limits.