[smux] 64-bit fix for lib/smux.h SNMP_INTEGER() macro

[smux] 64-bit fix for lib/smux.h SNMP_INTEGER() macro

Macro SNMP_INTEGER() prepares data which is eventually processed
by asn_build_int().  SNMP_INTEGER() was using "int32_t" whereas
asn_build_int() uses "long".  On 32-bit systems these are the
same, both 4 bytes, but on x86 64-bit systems "long" is 8 bytes.

asn_build_int()'s reaction to an improperly sized value is to return
a NULL pointer.  Quagga's smux.c would eventually get this NULL
pointer and use it in calculations to determine how much data to send
over the smux connection, resulting in garbage being sent to the
SNMP agent.

Corrected SNMP_INTEGER() to use "long".

Tested on 32-bit and 64-bit x86 Linux 2.6.27.10 systems running
Quagga 0.99.11 with bgpd smux.