opus/dec_celt: avoid emph_coeff becoming a subnormal

This happens for silence frames, which on many CPUs massively slows down
processing the decoded output.

Cf. https://github.com/Genymobile/scrcpy/issues/6715

(cherry picked from commit 5b112b17c0)

libavcodec/opus/dec_celt.c

@@ -463,6 +463,8 @@ int ff_celt_decode_frame(CeltFrame *f, OpusRangeCoder *rc,
                                                   block->emph_coeff,
                                                   ff_opus_deemph_weights,
                                                   frame_size);
+        if (!isnormal(block->emph_coeff))
+            block->emph_coeff = 0.0;
     }
 
     if (channels == 1)