libc: handle zero alignment in memalign()

For compatibility with glibc. The previous code would trigger a division
by zero in roundup() and terminate. Instead, just pass through to
malloc() for align == 0.

PR: 269688
Reviewed by: imp, mjg
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/655

(cherry picked from commit 2c709ee70ade9fd8f77b37917a4169d667dda41d)