include: fix compiler warning

include: fix compiler warning

./include/optutils.h:12:18: warning: null pointer dereference [-Wnull-dereference]
  for (o = opts; o->name; o++)
                 ~^~~~~~
In file included from libfdisk/src/dos.c:12:0:
./include/pt-mbr.h:25:47: warning: potential null pointer dereference [-Wnull-dereference]
  return p[0] | (p[1] << 8) | (p[2] << 16) | (p[3] << 24);
         ~^~~

Well these should be impossible, so add assert() to catch possible bugs.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>