Added some null checks after talloc_zero

We've seen the scenario with proxy-inner-tunnel as virtual server for PEAP, and a backend that is unreachable. With a storm of requests this did lead to segfaults in `session_init` (`src/main/tls.c`). From there, it's just turtles all the way down.
Merge pull request #860 from qnet-herwin/talloc_zero_null_checks

Added some null checks after talloc_zero
Remove URI field from rlm_ldap
We now use the correct name for function macros...
Fix more issues in LDAP url handling
Don't overwrite ip_hton af/prefix in fr_pton4

Don't overwrite address family and prefix set by ip_hton (which can fall
back onto IPv6) with AF_INT/32, in fr_pton4.

This fixes radiusd listening on IPv4 address consiting of the first 4
octets of resolved IPv6 address when falling back to IPv6.
raddb: Comment on ipaddr/ipv4addr/ipv6addr use

Describe combined ipaddr/ipv4addr/ipv6addr use in
raddb/sites-available/default.
same change for pton6
Typo