Fix bugs reported by coverity scan

Fix bugs reported by coverity scan

1. Fix useless call in bgpd/bgp_mplsvpn.c
Coverity scan ID : 1302528. Calling "decode_label(pnt)" is only useful
for its return value, which is
ignored. Removed the call.

2. Fix logically dead code in lib/stream.c
Coverity Scan ID 1302488. Test for size==0 makes no sense, since assert
immediately before it
would not let this code happen.

3. Fix Free Pointer dereference in lib/filter.c
Coverity Scan ID 23056. access is accessed after free in
access_list_delete