The chroma part of pal_idx potentially conflicts during intra
reconstruction with edge_{8,16}bpc. Fixes out of range pixel values
caused by invalid palette indices in
clusterfuzz-testcase-minimized-dav1d_fuzzer_mt-5076736684851200.
Fixes #294. Reported as integer overflows in boxsum5sqr with undefined
behavior sanitizer. Credits to oss-fuzz.