The regex engine is prone to excessive backtracking, leading to
timeouts, especially while fuzzing.
This commit introduces a backtracking counter and a limit of 1000
backtracking steps. When this limit is exceeded during fuzzing, the
regex engine aborts to prevent excessive backtracking. For this, the
FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION macro is used, as per
suggested by the documentation of libFuzzer.
- add inline function to test and convert surrogates
is_surrogate(c), is_hi_surrogate(c), is_lo_surrogate(c),
get_hi_surrogate(c), get_lo_surrogate(c), from_surrogate(hi, lo)
- use names for BC header offsets and lengths in libregexp.c
- remove strict aliasing violations in `lre_exec_backtrack()`
- pass all context variables to XXX_CHAR macros in `lre_exec_backtrack()`
