avcodec/mlpdec: Fix runtime error: shift exponent -5 is negative

Fixes part of 1708/clusterfuzz-testcase-minimized-5035111957397504

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

libavcodec/mlpdec.c

@@ -861,8 +861,13 @@ static int read_decoding_params(MLPDecodeContext *m, GetBitContext *gbp,
 
     if (s->param_presence_flags & PARAM_OUTSHIFT)
         if (get_bits1(gbp)) {
-            for (ch = 0; ch <= s->max_matrix_channel; ch++)
+            for (ch = 0; ch <= s->max_matrix_channel; ch++) {
                 s->output_shift[ch] = get_sbits(gbp, 4);
+                if (s->output_shift[ch] < 0) {
+                    avpriv_request_sample(m->avctx, "Negative output_shift");
+                    s->output_shift[ch] = 0;
+                }
+            }
             if (substr == m->max_decoded_substream)
                 m->dsp.mlp_pack_output = m->dsp.mlp_select_pack_output(s->ch_assign,
                                                                        s->output_shift,