Core: fixed port handling in ngx_parse_inet6_url().

This fixes buffer over-read when no port is specified in cases
similar to 5df5d7d771f6, and catches missing port separator.

src/core/ngx_inet.c

diff --git a/src/core/ngx_inet.c b/src/core/ngx_inet.c
index 33b303d..d5b7cf9 100644
--- a/src/core/ngx_inet.c
+++ b/src/core/ngx_inet.c
@@ -861,7 +861,12 @@
             last = uri;
         }
 
-        if (*port == ':') {
+        if (port < last) {
+            if (*port != ':') {
+                u->err = "invalid host";
+                return NGX_ERROR;
+            }
+
             port++;
 
             len = last - port;