make prefix_path() never return NULL

make prefix_path() never return NULL

There are 9 places where prefix_path is called, and only in one of
them the returned pointer was checked to be non-zero and only to
call exit(128) as it is usually done by die(). In other 8 places,
the returned value was not checked and it caused SIGSEGV when a
path outside of the working tree was used. For instance, running
  git update-index --add /some/path/outside
caused SIGSEGV.

This patch changes prefix_path() to die if the path is outside of
the repository, so it never returns NULL.

Signed-off-by: Dmitry Potapov <dpotapov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>