Return NULL early in context_preallocated_create if flags invalid

Return NULL early in context_preallocated_create if flags invalid

If the user passes invalid flags to _context_create, and the default
illegal callback does not abort the program (which is possible), then we
work with the result of malloc(0), which may be undefined behavior. This
violates the promise that a library function won't crash after the
illegal callback has been called.

This commit fixes this issue by returning NULL early in _context_create
in that case.