[libc] fix strtol returning the wrong length

Previously, strtol/ll/ul/ull would return a pointer to the end of its
parsing, regardless of if it detected a number. Now it will return a
length of 0 when it doesn't find a number.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D112176

GitOrigin-RevId: 53804d4eb2866b4741ebf6c76a8bee8580dde259
5 files changed