PR 117812 reports that testing GCC with zstd 1.3.4 fails because
ZSTD_CLEVEL_DEFAULT is not defined, so avoid using it.
PR libbacktrace/117812
* zstdtest.c (test_large): Use 3 rather than ZSTD_CLEVEL_DEFAULT
zstdtest has some inline data where some testcases lack the
uncompressed length field. Thus it computes that but still
ends up allocating memory for the uncompressed buffer based on
that (zero) length. Oops. Causes memory corruption if the
allocator returns non-NULL.
libbacktrace/
* zstdtest.c (test_samples): Properly compute the allocation
size for the uncompressed data.