Fix crash in v4l2 plugin.

This adds checks to make sure munmap and bfree are only called
when there was actually memory mapped and allocated.

plugins/linux-v4l2/v4l2-helpers.c

@@ -111,12 +111,14 @@ int_fast32_t v4l2_create_mmap(int_fast32_t dev, struct v4l2_buffer_data *buf)
 int_fast32_t v4l2_destroy_mmap(struct v4l2_buffer_data *buf)
 {
 	for(uint_fast32_t i = 0; i < buf->count; ++i) {
-		if (buf->info[i].start != MAP_FAILED)
+		if (buf->info[i].start != MAP_FAILED && buf->info[i].start != 0)
 			v4l2_munmap(buf->info[i].start, buf->info[i].length);
 	}
 
-	buf->count = 0;
-	bfree(buf->info);
+	if (buf->count) {
+		bfree(buf->info);
+		buf->count = 0;
+	}
 
 	return 0;
 }