ICU-21492 Fix regex compile assertion failure.

ICU-21492 Fix regex compile assertion failure.

A regex pattern containing nested look-behind blocks could trigger an assertion
failure during pattern compilation. The problem was caused by an off-by-one
error in the code that computes an upper bound on the match length, needed
because look-behind expressions are constrained to not have unbounded match
length.

Nested look-behind blocks come into play because, when computing the maximum
match length of an outer block, any inner look-behind blocks are skipped over -
they do not directly contribute to the length matched by the outer block. The
problem was in the code that skips over these nested look-behind blocks.