DEADLYSIGNAL when compress into BC3 format

[frokaikan]

I use cmake to build the library then run:
./img2ktx -o out.ktx -f BC3 input/not_kitty.png
here's not_kitty.png:
not_kitty.zip

Loaded input/not_kitty.png -- width=32 height=32 comp=3
compressing mip 0 layer 0: width=32 height=32 pitch_x=32 pitch_y=32
AddressSanitizer:DEADLYSIGNAL
=================================================================
==3641==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x0000008c0f2a bp 0x7ffd3a604c00 sp 0x7ffd3a604778 T0)
==3641==The signal is caused by a READ memory access.
==3641==Hint: address points to the zero page.
    #0 0x8c0f29 in _fini (/opt/img2ktx/img2ktx+0x8c0f29)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/opt/img2ktx/img2ktx+0x8c0f29) in _fini
==3641==ABORTING

[cdwfs]

I'm afraid I don't have a Unix system to test on right now, and running on Windows seems to produce the expected output successfully. However, there's a decent chance that whatever ASan is complaining about is in ispc_texcomp, and I happen to be in the middle of replacing the ispc_texcomp compression backend with AMD's Compressonator. If you're curious (and haven't lost interest or found an alternative tool by now -- sorry for the delay in investigating!), I can point you towards the branch where this work is happening and you can see if the problem has disappeared along with the old backend.

[cdwfs]

This will likely be fixed by #6 once the branch is merged