TRAP-TYPE definitions cause false positives in Mib#getSymbolByOid(String oid)

[derekm]

If a loaded MIB has TRAP-TYPE definitions, they act as a catch-all when using getSymbolByOid(String oid).

This prohibits loading several MIBs and looping over them in a getMibForOid(String oid) routine without editing the MIB and removing the TRAP-TYPE definitions or without further inspecting and rejecting the candidate symbol.