avcodec/takdec: Fix runtime error: signed integer overflow: 2146548196 + 2156738 cannot be represented in type 'int'

Fixes: 1743/clusterfuzz-testcase-minimized-4994834022531072

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

libavcodec/takdec.c

@@ -203,7 +203,7 @@ static void decode_lpc(int32_t *coeffs, int mode, int length)
         return;
 
     if (mode == 1) {
-        int a1 = *coeffs++;
+        unsigned a1 = *coeffs++;
         for (i = 0; i < length - 1 >> 1; i++) {
             *coeffs   += a1;
             coeffs[1] += *coeffs;
@@ -213,14 +213,14 @@ static void decode_lpc(int32_t *coeffs, int mode, int length)
         if (length - 1 & 1)
             *coeffs += a1;
     } else if (mode == 2) {
-        int a1    = coeffs[1];
-        int a2    = a1 + *coeffs;
+        unsigned a1    = coeffs[1];
+        unsigned a2    = a1 + *coeffs;
         coeffs[1] = a2;
         if (length > 2) {
             coeffs += 2;
             for (i = 0; i < length - 2 >> 1; i++) {
-                int a3    = *coeffs + a1;
-                int a4    = a3 + a2;
+                unsigned a3    = *coeffs + a1;
+                unsigned a4    = a3 + a2;
                 *coeffs   = a4;
                 a1        = coeffs[1] + a3;
                 a2        = a1 + a4;
@@ -231,13 +231,13 @@ static void decode_lpc(int32_t *coeffs, int mode, int length)
                 *coeffs += a1 + a2;
         }
     } else if (mode == 3) {
-        int a1    = coeffs[1];
-        int a2    = a1 + *coeffs;
+        unsigned a1    = coeffs[1];
+        unsigned a2    = a1 + *coeffs;
         coeffs[1] = a2;
         if (length > 2) {
-            int a3  = coeffs[2];
-            int a4  = a3 + a1;
-            int a5  = a4 + a2;
+            unsigned a3  = coeffs[2];
+            unsigned a4  = a3 + a1;
+            unsigned a5  = a4 + a2;
             coeffs[2] = a5;
             coeffs += 3;
             for (i = 0; i < length - 3; i++) {