avcodec/alsdec: Avoid dereferencing context pointer in inner interleave loop

This makes the decoder faster

Improves/Fixes: Timeout (22sec -> 20sec)
Testcase: 17619/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5078510820917248

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 581a895c5c)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/alsdec.c

@@ -1568,15 +1568,17 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame_ptr,
     #define INTERLEAVE_OUTPUT(bps)                                                   \
     {                                                                                \
         int##bps##_t *dest = (int##bps##_t*)frame->data[0];                          \
+        int channels = avctx->channels;                                              \
+        int32_t **raw_samples = ctx->raw_samples;                                    \
         shift = bps - ctx->avctx->bits_per_raw_sample;                               \
         if (!ctx->cs_switch) {                                                       \
             for (sample = 0; sample < ctx->cur_frame_length; sample++)               \
-                for (c = 0; c < avctx->channels; c++)                                \
-                    *dest++ = ctx->raw_samples[c][sample] * (1U << shift);            \
+                for (c = 0; c < channels; c++)                                       \
+                    *dest++ = raw_samples[c][sample] * (1U << shift);                \
         } else {                                                                     \
             for (sample = 0; sample < ctx->cur_frame_length; sample++)               \
-                for (c = 0; c < avctx->channels; c++)                                \
-                    *dest++ = ctx->raw_samples[sconf->chan_pos[c]][sample] * (1U << shift); \
+                for (c = 0; c < channels; c++)                                       \
+                    *dest++ = raw_samples[sconf->chan_pos[c]][sample] * (1U << shift);\
         }                                                                            \
     }