Fix out of bounds memory read in `add_compile_string`

[sashashura]

This PR fixes out of bounds memory read in add_compile_string revealed by fuzzing fluent-bit: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=46086

The root cause is that a call to enclen in compile_string_node results in a call to onigenc_mbclen_approximate. When the value of p passed to the function is \xf2 even though it is the last byte in multibyte sequince (the next byte is unexpected string terminator \0) the onigenc_mbclen_approximate returns it's size as 4. The size is added to the overall string length and results in reading past the end of the string.