wpa/hostapd: Fix 100% CPU when USB wlan NIC removed

wpa/hostapd: Fix 100% CPU when USB wlan NIC removed

hostapd calls pcap_next(3) to read the next packet off the wlan interface.
pcap_next() returns a pointer to the packet header but does not indicate
success or failure. Unfortunately this results in an infinite loop (100%
CPU) when the wlan device disappears, i.e. when a USB wlan device is
manually removed or a USB error results in the device removal. However
pcap_next_ex(3) does return success or failure. To resolve this we use
pcap_next_ex(), forcing hostapd to exit when the error is encountered.

An error message is printed to syslog or stderr when debugging (-d flag)
is enabled. Unfortunately wpa_printf() only works when debugging is enabled.

PR:		253608
Reported by:	Damjan Jovanovic <damjan.jov@gmail.com>,
		bz (privately)
MFC after:	3 days