library/string.m:
Make string.prev_index and string.unsafe_prev_index
return either U+FFFD or an unpaired surrogate code point
when an ill-formed code unit sequence is detected.
tests/hard_coded/Mmakefile:
tests/hard_coded/string_prev_index_ilseq.exp:
tests/hard_coded/string_prev_index_ilseq.exp2:
tests/hard_coded/string_prev_index_ilseq.m:
Add test case.
