swscale/vscale: Check that 2 tap filters are bilinear before using bilinear code

Fixes: out of array reads
Fixes: 07e8b9c5d348ccdf7add0f37de20cf6c/asan_heap-oob_27e8df7_6849_e56653f768070ec8cb52f587048444c2.mov

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
