zstd decompression helper cleanups

Rename the function for consistency with _openslide_inflate_buffer().
Don't bother checking the size of the first compressed frame, since there
might be more than one, and libzstd should fail if there isn't enough
output space.  Do check that the decompressed data matches the expected
length.  Use int64_t arguments rather than ones with arch-dependent
widths.  Use g_try_malloc().

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>