Fix some minor undefined behaviours #1810
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The bit-wise AND, OR and XOR logic works on the overloaded double (res->d), but casting this to int64_t for NAN is undefined behaviour. Infact the current cast doesn't even yield the correct bit-wise layout for NAN, as (long)d and *(long *)&d give different results for d==NAN. However the result wasn't used anyway as it's promptly cleared again in the subsequent `if (undef) hts_expr_val_undef(res)` code. Hence undefined or otherwise, it made no difference.
Obviously there can still be overflows if we attempt to parse numbers which are too big, but for the legally accepted range of this, parsing "-9,223,372,036,854,775,808" as tested in test/sam.c triggered a problem as the positive version doesn't fit in "long long". We parse as unsigned and only switch to signed via the implicit return type conversion (and probably exploiting twos-complement, but that's a fair assumption).
The pointer was never used, but the NULL+0 still triggers clang's ubsan.
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As reported by clang (gcc finds fewer).
Ie nasal demon avoidance.