tell ubsan to ignore SUN_LEN

ubsan will report undefined behavior due to the SUN_LEN macros interaction with a null pointer,
so let's tell ubsan to ignore this function. After carefully reviewing the final publically
availlable drafts of the C99,C11 and C18 standards I can confirm that dereferencing null pointers
is still undefined behavior, as such ubsan will always warn with absolutely every existing compiler
version. Since the sanitizers are periodically synced between llvm and gcc I'm also fairly confident
that rebuilding everything with compiler_rt to use the integrated sanitizers would result in the same message.
I sincerly hope that this explanation provides to be sufficient, If not I'd be willing to show up at
the next llvm dev meeting to provide quotes from actual sanitizer developers to back up these claims.

Change-Id: I0ff445072f1b46390c9f70b21d61c789e39358d5
diff --git a/src/socket.c b/src/socket.c
index 1e303bf..c50af55 100644
--- a/src/socket.c
+++ b/src/socket.c
@@ -614,6 +614,9 @@
  * type and \a proto and optionally binds or connects it, depending on
  * the value of \a flags parameter.
  */
+#if defined(__clang__) && defined(SUN_LEN)
+__attribute__((no_sanitize("undefined")))
+#endif
 int osmo_sock_unix_init(uint16_t type, uint8_t proto,
 			const char *socket_path, unsigned int flags)
 {