[libc++] Fix incorrect length check in std::basic_filebuf

This patch fixes an ASAN-found issue in std::basic_filebuf where we'd
check the wrong size before proceeding to set our internal buffer to
the externally-provided buffer, leading to the library trying to read
from the incorrect buffer in underflow().

Thanks to Andrey Semin for the patch.

Differential Revision: https://reviews.llvm.org/D154514