swscale/x86/rgb2rgb_template: Fix planar2x() for short width

Fixes: 451b3e0cf956c0bd2f27ed753ac24050/asan_heap-oob_2873c01_3231_7ed10a9464d15f0d57277f5917c566a8.AVI

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