[libcxx] Check return value for asprintf()

local __libcpp_asprintf_l() -> libc asprintf() was inspecting the pointer (with
indeterminate value) for failure, rather than the return value of -1.

Reviewed By: ldionne

Differential Revision: https://reviews.llvm.org/D94564

GitOrigin-RevId: 1be2524b7d213035e591bee3eecccdd6b59d14a5
1 file changed