Fix a compilation warning introduced by clang 3.6

There was a dead check when we made sure that an array member of a
struct was non-NULL.  Tor has been doing this check since at least
0.2.3, maybe earlier.

Fixes bug 17781.

changes/bug17781

@@ -0,0 +1,3 @@
+  o Compilation fixes:
+    - Fix a compilation warning with Clang 3.6: Do not check the
+      presence of an address which can never be NULL. Fixes bug 17781.

src/or/connection_edge.c

@@ -748,7 +748,7 @@ connection_ap_fail_onehop(const char *failed_digest,
       /* we don't know the digest; have to compare addr:port */
       tor_addr_t addr;
       if (!build_state || !build_state->chosen_exit ||
-          !entry_conn->socks_request || !entry_conn->socks_request->address)
+          !entry_conn->socks_request)
         continue;
       if (tor_addr_parse(&addr, entry_conn->socks_request->address)<0 ||
           !tor_addr_eq(&build_state->chosen_exit->addr, &addr) ||