dec_free_mi: Reset cm->mi_alloc_size.
dec_free_mi: Reset cm->mi_alloc_size.

That way, if one call to av1_dec_alloc_mi() fails, then
cm->mi_alloc_size is reset correctly, so that next call to
av1_dec_alloc_mi() can work correctly.

Note: The corresponding function on encoder side enc_free_mi() does
this already.

BUG=oss-fuzz:9268

Change-Id: Iaa7e0c43cce3b9f19a16ea5b09ba85f944be35ad

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom