[ttgxvar] Check for overflow in array size computation.

Problem reported and analyzed by povcfe <povcfe2sec@gmail.com>.

Fixes issue #1382.

* src/truetype/ttgxvar.c (tt_var_load_item_variation_store): Do it.

Bug: 503542587
Test: locally built
Flag: EXEMPT CVE_FIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:e6bcebe7f6e01c36fd7334d31eb0fa531e822ee9
Merged-In: I71441187859a413c0094637f7e9cfead76616a95
Change-Id: I71441187859a413c0094637f7e9cfead76616a95