Avoid signed overflow in AndroidZoneInfoSource::Open offset math

[ravi0800]

AndroidZoneInfoSource::Open() computes a zone's file position as data_offset + Decode32(ebuf + 40), where both operands are int_fast32_t values pulled straight from the tzdata index. On a build where int_fast32_t is a 32-bit type (32-bit Android/ARM), two near-INT_MAX index values sum past the type's range, so the addition is signed overflow before the start < 0 guard on the next line ever runs. UBSan on a crafted tzdata blob reports "signed integer overflow: 76 + 2147483647 cannot be represented in type int" right at that add.

Widen the left operand to int_fast64_t so the sum is formed in a type that holds it, which leaves the existing start < 0 / length < 0 rejection meaningful and the later static_cast(start) for fseek unchanged. Doing the widening at the add keeps the fix inside the decode where the operand widths are still in view.

[devbww]

This looks like it is being handled in #356. Please take a look there and see if you have anything to add. Otherwise we can close this PR. Thanks.