tinfl_decompress_mem_to_heap fails to decompress any input due to early exit in tinfl_decompress

[phildremi]

In fe6330e1d8b975ff075ab3bd85709480727e8afa, @jpcha2 added an early exit to tinfl_decompress. Unfortunately, this broke tinfl_decompress_mem_to_heap:

• When tinfl_decompress_mem_to_heap is first called, the output buffer is not yet allocated (NULL)
• tinfl_decompress receives this value and now exits early with error code TINFL_STATUS_BAD_PARAM
• This causes tinfl_decompress_mem_to_heap to return NULL as well, without any decompression having taken place
• Before the change, this line would have been reached to allocate an output buffer of 128 bytes instead

If you run the miniz_tester app before/after the change, you should be able to see it fail in simple_test1.