socket: Prevent buffer under-read in nxt_inet_addr()

This was found via ASan.

Given a listener address like ":" (or any address where the first
character is a colon) we can end up under-reading the addr->start
buffer here

  if (nxt_slow_path(*(buf + length - 1) == '.')) {

due to length (essentially the position of the ":" in the string) being
0.

Seeing as any address that starts with a ":" is invalid Unit config
wise, we should simply reject the address if length == 0 in
nxt_sockaddr_inet_parse().

Link: <https://clang.llvm.org/docs/AddressSanitizer.html>
Signed-off-by: Arjun <pkillarjun@protonmail.com>
[ Commit message - Andrew ]
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Author: pkillarjun

src/nxt_sockaddr.c

@@ -732,6 +732,11 @@ nxt_sockaddr_inet_parse(nxt_mp_t *mp, nxt_str_t *addr)
         length = p - addr->start;
     }
 
+    if (length == 0) {
+        nxt_thread_log_error(NXT_LOG_ERR, "invalid address \"%V\"", addr);
+        return NULL;
+    }
+
     inaddr = INADDR_ANY;
 
     if (length != 1 || addr->start[0] != '*') {