[libc] fix strtointeger hex prefix parsing

Fix edge case where "0x" would be considered a complete hexadecimal
number for purposes of str_end. Now the hexadecimal prefix needs a valid
digit after it, else just the 0 will be counted as the number.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D109084

GitOrigin-RevId: 37ce7349f7e95ef19cd6169e70414286b33114cb
5 files changed