Fix signed integer overflow

[MichaelChirico]

This was an error produced by a fuzzer we set up for yajl, fixed by Gemini, and ported to this bundled version by Gemini.

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yajl_parse_integer was susceptible to signed integer overflow when parsing numbers close to or exceeding LLONG_MAX/LLONG_MIN limits. It performed multiplication and addition before checking if the result would overflow.

Solution:
Refactor the parsing loop in yajl_parse_integer to check for potential overflow before modifying the accumulator:

• For positive numbers, check if multiplying by 10 or adding the next digit would exceed LLONG_MAX.
• For negative numbers, accumulate as a negative value directly (to correctly handle LLONG_MIN which has larger absolute value than LLONG_MAX) and check if multiplying by 10 or subtracting the next digit would exceed LLONG_MIN.
• Set errno to ERANGE and return the capped value (LLONG_MAX/LLONG_MIN) upon overflow, allowing the YAJL tree parser to fall back to double precision representation.

[MichaelChirico]

We could also use __builtin_mul_overflow here for a very clean solution, which is supported on clang and gcc, but not everywhere:

https://clang.llvm.org/docs/LanguageExtensions.html#checked-arithmetic-builtins