arm: fix use of uninitialised value in ff_fft_fixed_init_arm()

When initialising an FFTContext for a plain FFT, mdct_bits is not set
and can contain a garbage value.  Since nbits is always valid and for
MDCT operation is mdct_bits - 2 checking this instead avoids using an
uninitialised value while having the same effect.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/arm/fft_fixed_init_arm.c

@@ -36,7 +36,7 @@ av_cold void ff_fft_fixed_init_arm(FFTContext *s)
         s->fft_calc        = ff_fft_fixed_calc_neon;
 
 #if CONFIG_MDCT
-        if (!s->inverse && s->mdct_bits >= 5) {
+        if (!s->inverse && s->nbits >= 3) {
             s->mdct_permutation = FF_MDCT_PERM_INTERLEAVE;
             s->mdct_calc        = ff_mdct_fixed_calc_neon;
             s->mdct_calcw       = ff_mdct_fixed_calcw_neon;