ip: ipmaddr.c: Fix possible integer underflow in read_igmp()

ip: ipmaddr.c: Fix possible integer underflow in read_igmp()

Static analyzer pointed out a potential error:

	Possible integer underflow: left operand is tainted. An integer underflow
	may occur due to arithmetic operation (unsigned subtraction) between variable
	'len' and value '1', when 'len' is tainted { [0, 18446744073709551615] }

The fix adds a check for 'len == 0' before accessing the last character of
the name, and skips the current line in such cases to avoid the underflow.

Reported-by: SVACE static analyzer
Signed-off-by: Anton Moryakov <ant.v.moryakov@gmail.com>
Signed-off-by: David Ahern <dsahern@kernel.org>