Don't leak string in parse_define(..) when a macro has been found

and the parser token != TOK_ASSIGN

MFC after: 1 week
CID: 1007187, 1007188
Reported by: Coverity
Obtained from: Isilon OneFS (part of r445479)
Submitted by: Miles Ohlrich <miles.ohlrich@isilon.com>
Sponsored by: EMC / Isilon Storage Division

contrib/bsnmp/snmpd/config.c

@@ -1150,7 +1150,8 @@ parse_define(const char *varname)
 			free(m->value);
 			m->value = string;
 			m->length = length;
-		}
+		} else
+			free(string);
 	}
 
 	token = TOK_EOL;