Fixed handling of non-null-terminated unix sockets.

At least FreeBSD, macOS, NetBSD, and OpenBSD can return unix sockets
with non-null-terminated sun_path.  Additionally, the address may become
non-null-terminated if it does not fit into the buffer provided and was
truncated (may happen on macOS, NetBSD, and Solaris, which allow unix socket
addresess larger than struct sockaddr_un).  As such, ngx_sock_ntop() might
overread the sockaddr provided, as it used "%s" format and thus assumed
null-terminated string.

To fix this, the ngx_strnlen() function was introduced, and it is now used
to calculate correct length of sun_path.

src/core/ngx_inet.c

diff --git a/src/core/ngx_inet.c b/src/core/ngx_inet.c
index 3bcd3e7..de68187 100644
--- a/src/core/ngx_inet.c
+++ b/src/core/ngx_inet.c
@@ -241,7 +241,9 @@
             p = ngx_snprintf(text, len, "unix:%Z");
 
         } else {
-            p = ngx_snprintf(text, len, "unix:%s%Z", saun->sun_path);
+            n = ngx_strnlen((u_char *) saun->sun_path,
+                            socklen - offsetof(struct sockaddr_un, sun_path));
+            p = ngx_snprintf(text, len, "unix:%*s%Z", n, saun->sun_path);
         }
 
         /* we do not include trailing zero in address length */

src/core/ngx_string.c

diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index de10a06..2ee07bf 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -29,6 +29,22 @@
 }
 
 
+size_t
+ngx_strnlen(u_char *p, size_t n)
+{
+    size_t  i;
+
+    for (i = 0; i < n; i++) {
+
+        if (p[i] == '\0') {
+            return i;
+        }
+    }
+
+    return n;
+}
+
+
 u_char *
 ngx_cpystrn(u_char *dst, u_char *src, size_t n)
 {

src/core/ngx_string.h

diff --git a/src/core/ngx_string.h b/src/core/ngx_string.h
index 7363bd2..882ae7c 100644
--- a/src/core/ngx_string.h
+++ b/src/core/ngx_string.h
@@ -60,6 +60,8 @@
 #define ngx_strstr(s1, s2)  strstr((const char *) s1, (const char *) s2)
 #define ngx_strlen(s)       strlen((const char *) s)
 
+size_t ngx_strnlen(u_char *p, size_t n);
+
 #define ngx_strchr(s1, c)   strchr((const char *) s1, (int) c)
 
 static ngx_inline u_char *