Use calloc to allocate xports (BZ #17542)

Use calloc to allocate xports (BZ #17542)

If xports is NULL in xprt_register we malloc it but if sock >
_rpc_dtablesize() that memory does not get initialised and may in theory
contain any value. Later we make a conditional jump in svc_getreq_common
based on the uninitialised memory and this caused a general protection
fault in rpc.statd on an older version of glibc but this code has not
changed since that version.

Following is the valgrind warning.

==26802== Conditional jump or move depends on uninitialised value(s)
==26802==    at 0x5343A25: svc_getreq_common (in /lib64/libc-2.5.so)
==26802==    by 0x534357B: svc_getreqset (in /lib64/libc-2.5.so)
==26802==    by 0x10DE1F: ??? (in /sbin/rpc.statd)
==26802==    by 0x10D0EF: main (in /sbin/rpc.statd)
==26802==  Uninitialised value was created by a heap allocation
==26802==    at 0x4C2210C: malloc (vg_replace_malloc.c:195)
==26802==    by 0x53438BE: xprt_register (in /lib64/libc-2.5.so)
==26802==    by 0x53450DF: svcudp_bufcreate (in /lib64/libc-2.5.so)
==26802==    by 0x10FE32: ??? (in /sbin/rpc.statd)
==26802==    by 0x10D13E: main (in /sbin/rpc.statd)