Added a preg_replace to get rid of snmp errors for bgp graphs

This is a bit of a work around but without refactoring how snmp is called then this is the best I can do. It's currently a bug and stops bgp graphs working for devices that don't support all currently used snmp calls:

This is currently what is happening:

RRD[update /opt/librenms/rrd/router/cbgp-1.1.1.1.ipv4.unicast.rrd N:0:No Such Instance currently exists at this OID:0:No Such Instance currently exists at this OID:No Such Instance currently exists at this OID]

This patch replaces No Such Instance currently exists at this OID with 0 so that an RRD update can happen and in turn generate the graphs.