avcodec/diracdec: Fix "warning: comparison of constant 4 with expression of...

avcodec/diracdec: Fix "warning: comparison of constant 4 with expression of type enum dirac_subband is always true [-Wtautological-constant-out-of-range-compare]"
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/diracdec.c

@@ -234,7 +234,8 @@ enum dirac_subband {
     subband_ll = 0,
     subband_hl = 1,
     subband_lh = 2,
-    subband_hh = 3
+    subband_hh = 3,
+    subband_nb,
 };
 
 static const uint8_t default_qmat[][4][4] = {