MemorySanitizer complained about uninitialized reads in
the indexed property code path in JS_GetPropertyValue()
with JS_CLASS_MAPPED_ARGUMENTS objects.
Compare against 0x1p63 instead of INT64_MAX. Converting INT64_MAX to
double rounds it up to INT64_MAX+1.
It made code like `if (d <= INT64_MAX) v = (int64_t)d;` behave subtly
wrong when `d >= 0x1p63` because then `v = (int64_t)d` wraps around to
a negative value.
