avcodec/pcx: Check for end of input in pcx_rle_decode() and propagate error

Fixes: Timeout
Fixes: 10279/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PCX_fuzzer-5729311395414016

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


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