Enable -Wnull-dereference (GCC >=6.1), and fix the easy cases

This warning, IIUC, means that the compiler doesn't like it when it
sees a NULL check _after_ we've already dereferenced the
variable. In such cases, it considers itself free to eliminate the
NULL check.

There are a couple of tricky cases:

One was the case related to the fact that tor_addr_to_in6() can
return NULL if it gets a non-AF_INET6 address.  The fix was to
create a variant which asserts on the address type, and never
returns NULL.

src/or/channeltls.c

@@ -797,6 +797,7 @@ static int
 channel_tls_write_packed_cell_method(channel_t *chan,
                                      packed_cell_t *packed_cell)
 {
+  tor_assert(chan);
   channel_tls_t *tlschan = BASE_CHAN_TO_TLS(chan);
   size_t cell_network_size = get_cell_network_size(chan->wide_circ_ids);
   int written = 0;

src/or/connection.c

@@ -2240,7 +2240,7 @@ connection_send_socks5_connect(connection_t *conn)
   } else { /* AF_INET6 */
     buf[3] = 4;
     reqsize += 16;
-    memcpy(buf + 4, tor_addr_to_in6(&conn->addr), 16);
+    memcpy(buf + 4, tor_addr_to_in6_addr8(&conn->addr), 16);
     memcpy(buf + 20, &port, 2);
   }
 

src/or/geoip.c

@@ -80,9 +80,9 @@ geoip_add_entry(const tor_addr_t *low, const tor_addr_t *high,
   intptr_t idx;
   void *idxplus1_;
 
-  if (tor_addr_family(low) != tor_addr_family(high))
+  IF_BUG_ONCE(tor_addr_family(low) != tor_addr_family(high))
     return;
-  if (tor_addr_compare(high, low, CMP_EXACT) < 0)
+  IF_BUG_ONCE(tor_addr_compare(high, low, CMP_EXACT) < 0)
     return;
 
   idxplus1_ = strmap_get_lc(country_idxplus1_by_lc_code, country);
@@ -110,8 +110,8 @@ geoip_add_entry(const tor_addr_t *low, const tor_addr_t *high,
     smartlist_add(geoip_ipv4_entries, ent);
   } else if (tor_addr_family(low) == AF_INET6) {
     geoip_ipv6_entry_t *ent = tor_malloc_zero(sizeof(geoip_ipv6_entry_t));
-    ent->ip_low = *tor_addr_to_in6(low);
-    ent->ip_high = *tor_addr_to_in6(high);
+    ent->ip_low = *tor_addr_to_in6_assert(low);
+    ent->ip_high = *tor_addr_to_in6_assert(high);
     ent->country = idx;
     smartlist_add(geoip_ipv6_entries, ent);
   }