new_dbus_handlers: Clear errno

new_dbus_handlers: Clear errno

There are a few instances where dbus handlers test the value
of errno to test whether strtoul completes successfully.
Since strtoul does not clear errno, and there's no strong
reason to suspect that errno is already clear, it is safer
to clear it right before calling strtoul.  Also, any failure
in strtoul (setting errno non-zero) should be considered a
failure.

While testing using dbus-send, I found that a malformed
network path can cause a crash due to net_id being left
NULL.  We should test for this before calling strtoul
on it.

Tested with:

dbus-send --system --dest=fi.w1.wpa_supplicant1 --print-reply \
    /fi/w1/wpa_supplicant1/Interfaces/0 \
    org.freedesktop.DBus.Properties.Get \
    string:fi.w1.wpa_supplicant1.Interface string:Networks
dbus-send --system --dest=fi.w1.wpa_supplicant1 --print-reply \
   /fi/w1/wpa_supplicant1/Interfaces/0 \
   fi.w1.wpa_supplicant1.Interface.RemoveNetwork \
   objpath:/fi/w1/wpa_supplicant1/Interfaces/0/Networks/0
dbus-send --system --dest=fi.w1.wpa_supplicant1 --print-reply \
   /fi/w1/wpa_supplicant1/Interfaces/0 \
   fi.w1.wpa_supplicant1.Interface.RemoveNetwork \
   objpath:/fi/w1/wpa_supplicant1/Interfaces/0/Networks/0
dbus-send --system --dest=fi.w1.wpa_supplicant1 --print-reply \
   /fi/w1/wpa_supplicant1/Interfaces/0 \
   fi.w1.wpa_supplicant1.Interface.RemoveNetwork \
   objpath:/fi/w1/wpa_supplicant1/Interfaces/0

Signed-hostap: Paul Stewart <pstew@chromium.org>
intended-for: hostap-1