avcodec/jpeg2000dwt: Check ndeclevels before calling dwt_decode*()

Fixes out of array access
Fixes: 01859c9a9ac6cd60a008274123275574/asan_heap-oob_1dff571_8250_50d3d1611e294c3519fd1fa82198b69b.avi

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/jpeg2000dwt.c

@@ -595,6 +595,9 @@ int ff_dwt_encode(DWTContext *s, void *t)
 
 int ff_dwt_decode(DWTContext *s, void *t)
 {
+    if (s->ndeclevels == 0)
+        return 0;
+
     switch (s->type) {
     case FF_DWT97:
         dwt_decode97_float(s, t);