Fix a segfault on *BSD

freeaddrinfo is not required by POSIX to be NULL-safe. OpenBSD will
SIGSEGV. NetBSD will assert. FreeBSD up to 11.1 will SIGSEGV, while in
future versions, it will be a silent NOP [1].

Commit d4b715f0aa ("Fix potential race in 'invalid timeout' tests")
added a code path to _redisContextConnectTcp which calls
freeaddrinfo(NULL), triggering the segfault. Put a NULL check around the
call to freeaddrinfo.

[1] - e916723903

Signed-off-by: Justin Brewer <jzb0012@auburn.edu>

net.c

@@ -412,7 +412,10 @@ addrretry:
 error:
     rv = REDIS_ERR;
 end:
+    if(servinfo) {
         freeaddrinfo(servinfo);
+    }
+
     return rv;  // Need to return REDIS_OK if alright
 }