libFLAC/stream_decoder: Fix double free

The american-fuzzy-lop fuzzer found a couple of instances of double
free() resulting from commit 15a9062609.

The problematic free() were the ones associated with use of the
safe_realloc_mul_2op_() function which can call realloc(ptr,0) which
according to the realloc manpage is already an implicit free().
1 file changed