avcodec/lsp: Fix undefined shifts in lsp2poly()

Fixes: left shift of negative value -30635
Fixes: 17689/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ACELP_KELVIN_fuzzer-5756275014500352

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 2b93f52cd6)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/lsp.c

@@ -108,7 +108,7 @@ static void lsp2poly(int* f, const int16_t* lsp, int lp_half_order)
     int i, j;
 
     f[0] = 0x400000;          // 1.0 in (3.22)
-    f[1] = -lsp[0] << 8;      // *2 and (0.15) -> (3.22)
+    f[1] = -lsp[0] * 256;     // *2 and (0.15) -> (3.22)
 
     for(i=2; i<=lp_half_order; i++)
     {
@@ -116,7 +116,7 @@ static void lsp2poly(int* f, const int16_t* lsp, int lp_half_order)
         for(j=i; j>1; j--)
             f[j] -= MULL(f[j-1], lsp[2*i-2], FRAC_BITS) - f[j-2];
 
-        f[1] -= lsp[2*i-2] << 8;
+        f[1] -= lsp[2*i-2] * 256;
     }
 }