bsd_user: Fix potential null pointer dereference

This bug was spotted by cppcheck.

Using g_try_malloc0 (as does the linux-user code) fixes this.

v2:
Use g_free in bsdload.c, too. Thanks to Peter Maydell for this hint.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
