Fix build with -DNDEBUG in CFLAGS

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/block/vvfat.c b/block/vvfat.c
index bb707c0..aaa8593 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -2462,10 +2462,13 @@
 	    fail = -2;
 	    break;
 	case ACTION_WRITEOUT: {
+#ifndef NDEBUG
+            /* these variables are only used by assert() below */
 	    direntry_t* entry = array_get(&(s->directory),
 		    commit->param.writeout.dir_index);
 	    uint32_t begin = begin_of_direntry(entry);
 	    mapping_t* mapping = find_mapping_for_cluster(s, begin);
+#endif
 
 	    assert(mapping);
 	    assert(mapping->begin == begin);