avutil/lfg: Correct index increment type to avoid undefined behavior

Fixes: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'
Fixes: 18333/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_COMFORTNOISE_fuzzer-5668481831272448

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 6014bcf1b7)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/lfg.h

@@ -36,8 +36,9 @@ void av_lfg_init(AVLFG *c, unsigned int seed);
  * it may be good enough and faster for your specific use case.
  */
 static inline unsigned int av_lfg_get(AVLFG *c){
-    c->state[c->index & 63] = c->state[(c->index-24) & 63] + c->state[(c->index-55) & 63];
-    return c->state[c->index++ & 63];
+    unsigned a = c->state[c->index & 63] = c->state[(c->index-24) & 63] + c->state[(c->index-55) & 63];
+    c->index += 1U;
+    return a;
 }
 
 /**
@@ -48,7 +49,9 @@ static inline unsigned int av_lfg_get(AVLFG *c){
 static inline unsigned int av_mlfg_get(AVLFG *c){
     unsigned int a= c->state[(c->index-55) & 63];
     unsigned int b= c->state[(c->index-24) & 63];
-    return c->state[c->index++ & 63] = 2*a*b+a+b;
+    a = c->state[c->index & 63] = 2*a*b+a+b;
+    c->index += 1U;
+    return a;
 }
 
 /**