avcodec/mdct_*: Fix integer overflow in addition in RESCALE()

Fixes: runtime error: signed integer overflow: 1219998458 - -1469874012 cannot be represented in type 'int'
Fixes: 3443/clusterfuzz-testcase-minimized-5369987105554432

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

libavcodec/mdct_fixed.c

@@ -39,13 +39,13 @@ void ff_mdct_calcw_c(FFTContext *s, FFTDouble *out, const FFTSample *input)
 
     /* pre rotation */
     for(i=0;i<n8;i++) {
-        re = RSCALE(-input[2*i+n3] - input[n3-1-2*i]);
-        im = RSCALE(-input[n4+2*i] + input[n4-1-2*i]);
+        re = RSCALE(-input[2*i+n3], - input[n3-1-2*i]);
+        im = RSCALE(-input[n4+2*i], + input[n4-1-2*i]);
         j = revtab[i];
         CMUL(x[j].re, x[j].im, re, im, -tcos[i], tsin[i]);
 
-        re = RSCALE( input[2*i]    - input[n2-1-2*i]);
-        im = RSCALE(-input[n2+2*i] - input[ n-1-2*i]);
+        re = RSCALE( input[2*i]   , - input[n2-1-2*i]);
+        im = RSCALE(-input[n2+2*i], - input[ n-1-2*i]);
         j = revtab[n8 + i];
         CMUL(x[j].re, x[j].im, re, im, -tcos[n8 + i], tsin[n8 + i]);
     }

libavcodec/mdct_template.c

@@ -33,12 +33,12 @@
  */
 
 #if FFT_FLOAT
-#   define RSCALE(x) (x)
+#   define RSCALE(x, y) ((x) + (y))
 #else
 #if FFT_FIXED_32
-#   define RSCALE(x) (((x) + 32) >> 6)
+#   define RSCALE(x, y) ((int)((x) + (unsigned)(y) + 32) >> 6)
 #else /* FFT_FIXED_32 */
-#   define RSCALE(x) ((x) >> 1)
+#   define RSCALE(x, y) ((int)((x) + (unsigned)(y)) >> 1)
 #endif /* FFT_FIXED_32 */
 #endif
 
@@ -181,13 +181,13 @@ void ff_mdct_calc_c(FFTContext *s, FFTSample *out, const FFTSample *input)
 
     /* pre rotation */
     for(i=0;i<n8;i++) {
-        re = RSCALE(-input[2*i+n3] - input[n3-1-2*i]);
-        im = RSCALE(-input[n4+2*i] + input[n4-1-2*i]);
+        re = RSCALE(-input[2*i+n3], - input[n3-1-2*i]);
+        im = RSCALE(-input[n4+2*i], + input[n4-1-2*i]);
         j = revtab[i];
         CMUL(x[j].re, x[j].im, re, im, -tcos[i], tsin[i]);
 
-        re = RSCALE( input[2*i]    - input[n2-1-2*i]);
-        im = RSCALE(-input[n2+2*i] - input[ n-1-2*i]);
+        re = RSCALE( input[2*i]   , - input[n2-1-2*i]);
+        im = RSCALE(-input[n2+2*i], - input[ n-1-2*i]);
         j = revtab[n8 + i];
         CMUL(x[j].re, x[j].im, re, im, -tcos[n8 + i], tsin[n8 + i]);
     }