pci: Fix crash with illegal "-net nic, model=xxx" option

Current QEMU crashes when specifying an illegal model with the
"-net nic,model=xxx" option, e.g.:

 $ qemu-system-x86_64 -net nic,model=n/a
 qemu-system-x86_64: Unsupported NIC model: n/a

 Program received signal SIGSEGV, Segmentation fault.

The gdb backtrace looks like this:

0x0000555555965fe0 in error_get_pretty (err=0x0) at util/error.c:152
152	    return err->msg;
(gdb) bt
 0  0x0000555555965fe0 in error_get_pretty (err=0x0) at util/error.c:152
 1  0x0000555555965ffd in error_report_err (err=0x0) at util/error.c:157
 2  0x0000555555809c90 in pci_nic_init_nofail (nd=0x555555e49860 <nd_table>, rootbus=0x5555564409b0,
    default_model=0x55555598c37b "e1000", default_devaddr=0x0) at hw/pci/pci.c:1663
 3  0x0000555555691e42 in pc_nic_init (isa_bus=0x555556f71900, pci_bus=0x5555564409b0)
    at hw/i386/pc.c:1506
 4  0x000055555569396b in pc_init1 (machine=0x5555562abbf0, pci_enabled=1, kvmclock_enabled=1)
    at hw/i386/pc_piix.c:248
 5  0x0000555555693d27 in pc_init_pci (machine=0x5555562abbf0) at hw/i386/pc_piix.c:310
 6  0x000055555572ddf5 in main (argc=3, argv=0x7fffffffe018, envp=0x7fffffffe038) at vl.c:4226

The problem is that pci_nic_init_nofail() does not check whether the err
parameter from pci_nic_init has been set up and thus passes a NULL pointer
to error_report_err(). Fix it by correctly checking the err parameter.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
