Fix assert with mz_zip_writer_add_mem* w/MZ_DEFAULT_COMPRESSION
Fix assert with mz_zip_writer_add_mem* w/MZ_DEFAULT_COMPRESSION

Issue #81

As MZ_DEFAULT_COMPRESSION is set as "-1", one of the bits is incorrectly
interpreted as the "MZ_ZIP_FLAG_COMPRESSED_DATA" flag before the check
for (flags < 0) intended to catch the MZ_DEFAULT_COMPRESION case.

This would then cause an assert() to be hit on any calls where the
uncomp_size is non-zero.

Moving the "Default" level check above any other flag checks should
solve this issue.

richgel999/miniz,richgel999/miniz,richgel999/miniz