avcodec/eac3dec: avoid float noise in fixed mode addition to overflow

Fixes: 2.28595e+09 is outside the range of representable values of type 'int'
Fixes: 54644/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AC3_FIXED_fuzzer-4816961584627712

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ac3.h

@@ -53,6 +53,7 @@
 #define AC3_DYNAMIC_RANGE1      0
 
 typedef int                     INTFLOAT;
+typedef unsigned int            UINTFLOAT;
 typedef int16_t                 SHORTFLOAT;
 
 #else /* USE_FIXED */
@@ -73,6 +74,7 @@ typedef int16_t                 SHORTFLOAT;
 #define AC3_DYNAMIC_RANGE1      1.0f
 
 typedef float                   INTFLOAT;
+typedef float                   UINTFLOAT;
 typedef float                   SHORTFLOAT;
 
 #endif /* USE_FIXED */

libavcodec/eac3dec.c

@@ -138,9 +138,11 @@ static void ff_eac3_apply_spectral_extension(AC3DecodeContext *s)
             // spx_noise_blend and spx_signal_blend are both FP.23
             nscale *= 1.0 / (1<<23);
             sscale *= 1.0 / (1<<23);
+            if (nscale < -1.0)
+                nscale = -1.0;
 #endif
             for (i = 0; i < s->spx_band_sizes[bnd]; i++) {
-                float noise  = nscale * (int32_t)av_lfg_get(&s->dith_state);
+                UINTFLOAT noise = (INTFLOAT)(nscale * (int32_t)av_lfg_get(&s->dith_state));
                 s->transform_coeffs[ch][bin]   *= sscale;
                 s->transform_coeffs[ch][bin++] += noise;
             }