Fixes: use of uninitialized memory in hScale16To15_c()
Fixes: 373924007/clusterfuzz-testcase-minimized-ffmpeg_SWS_fuzzer-5841199968092160
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>