avcodec/lagarith: Optimize FRAME_SOLID_RGBA

Changes 5466110 decicycles to 1798072 decicycles (tested with fate-suite/lagarith/lag-rgb32.avi)

Fixes: Timeout
Fixes: 9484/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_LAGARITH_fuzzer-5707859156271104

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>