hfpmodem: Fix segfault in CIEV GAtChat callback

hfpmodem: Fix segfault in CIEV GAtChat callback

This patch fixes segmentation fault when the network registration
watch is called without being initialized. CIEV GAtChat callback can
be called before ofono_netreg_register().

==15101== Invalid read of size 8
==15101==    at 0x492B56: ofono_netreg_register (network.c:2073)
==15101==    by 0x47245E: hfp_netreg_probe (network-registration.c:311)
==15101==    by 0x492A8D: ofono_netreg_create (network.c:1881)
==15101==    by 0x4849D5: hfp_pre_sim (hfp_hf_bluez5.c:288)
==15101==    by 0x48C486: ofono_modem_set_powered (modem.c:1194)
==15101==    by 0x484E9D: slc_established (hfp_hf_bluez5.c:85)
==15101==    by 0x4702AD: chld_cb (slc.c:147)
==15101==    by 0x440457: at_chat_finish_command (gatchat.c:461)
==15101==    by 0x44109F: new_bytes (gatchat.c:532)
==15101==    by 0x4433B7: received_data (gatio.c:122)
==15101==    by 0x3CBAA47824: g_main_context_dispatch (gmain.c:2539)
==15101==    by 0x3CBAA47B57: g_main_context_iterate.isra.23
(gmain.c:3146)
==15101==  Address 0x18 is not stack'd, malloc'd or (recently) free'd