Fix 32bit and 64bit counter selector

The script fails checking 64 bit counters:

$ ./check_traffic.sh -V 2c -C public  -H 192.168.1.89 -I 1 -w 200,200 -c 300,300 -K -B
IF-MIB::.1: Unknown Object Identifier
IF-MIB::.1: Unknown Object Identifier
No Data been get here. Please confirm your ARGS and re-check it with Verbose mode, then to check the log.(If you snmp not support 64 bit counter, do not use -6 option)

That's because snmpget exit code is not reliable:

$ /usr/bin/snmpget -t 15 -Oa -v 2c -c public 192.168.1.89 IF-MIB::ifHCOutOctets.1
IF-MIB::ifHCOutOctets.1 = No Such Object available on this agent at this OID
$ echo $?
0

$ /usr/bin/snmpget -t 15 -Oa -v 2c -c public 192.168.1.89 IF-MIB::ifOutOctets.1
IF-MIB::ifOutOctets.1 = Counter32: 4050020
$ echo $?
0

The patch checks the snmpget output instead of the error code in order to
choose a 32counter or a 64counter.
Merge pull request #2 from dgilm/fix-counter64-snmpget-error-code

Fix 32bit and 64bit counter selector