cleanup: Use static_assert instead of assert where possible.

This avoids some "always true condition" warnings and lifts the errors
(if any) into compile time.

other/bootstrap_daemon/docker/tox-bootstrapd.sha256

@@ -1 +1 @@
-96618672392dc44a2f0a480f6415380e3fc588ace646345d31a297e80f7c271f  /usr/local/bin/tox-bootstrapd
+60f76a2186014870804b8dbacc16c68e8b019e02699584d007327a72de9e53de  /usr/local/bin/tox-bootstrapd

testing/BUILD.bazel

@@ -18,7 +18,6 @@ sh_test(
         "-Wno-callgraph",
         "-Wno-enum-names",
         "-Wno-type-check",
-        "-Wno-var-unused-in-scope",
         "+RTS",
         "-N3",
         "-RTS",

toxcore/Messenger.c

@@ -2744,7 +2744,7 @@ non_null()
 static uint8_t *save_nospam_keys(const Messenger *m, uint8_t *data)
 {
     const uint32_t len = m_plugin_size(m, STATE_TYPE_NOSPAMKEYS);
-    assert(sizeof(get_nospam(m->fr)) == sizeof(uint32_t));
+    static_assert(sizeof(get_nospam(m->fr)) == sizeof(uint32_t), "nospam doesn't fit in a 32 bit int");
     data = state_write_section_header(data, STATE_COOKIE_TYPE, len, STATE_TYPE_NOSPAMKEYS);
     const uint32_t nospam = get_nospam(m->fr);
     host_to_lendian_bytes32(data, nospam);

toxcore/network.c

@@ -313,13 +313,16 @@ static const Family *make_tox_family(int family)
 non_null()
 static void get_ip4(IP4 *result, const struct in_addr *addr)
 {
+    static_assert(sizeof(result->uint32) == sizeof(addr->s_addr),
+                  "Tox and operating system don't agree on size of IPv4 addresses");
     result->uint32 = addr->s_addr;
 }
 
 non_null()
 static void get_ip6(IP6 *result, const struct in6_addr *addr)
 {
-    assert(sizeof(result->uint8) == sizeof(addr->s6_addr));
+    static_assert(sizeof(result->uint8) == sizeof(addr->s6_addr),
+                  "Tox and operating system don't agree on size of IPv6 addresses");
     memcpy(result->uint8, addr->s6_addr, sizeof(result->uint8));
 }
 
@@ -332,7 +335,6 @@ static void fill_addr4(const IP4 *ip, struct in_addr *addr)
 non_null()
 static void fill_addr6(const IP6 *ip, struct in6_addr *addr)
 {
-    assert(sizeof(ip->uint8) == sizeof(addr->s6_addr));
     memcpy(addr->s6_addr, ip->uint8, sizeof(ip->uint8));
 }