Index initialise the nxt_app_msg_prefix array.

This makes it much more clear what's what.

This is in preparation for adding WebAssembly language module support.

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

src/nxt_router.c

@@ -274,12 +274,12 @@ static const nxt_str_t http_prefix = nxt_string("HTTP_");
 static const nxt_str_t empty_prefix = nxt_string("");
 
 static const nxt_str_t  *nxt_app_msg_prefix[] = {
-    &empty_prefix,
-    &empty_prefix,
-    &http_prefix,
-    &http_prefix,
-    &http_prefix,
-    &empty_prefix,
+    [NXT_APP_EXTERNAL]  = &empty_prefix,
+    [NXT_APP_PYTHON]    = &empty_prefix,
+    [NXT_APP_PHP]       = &http_prefix,
+    [NXT_APP_PERL]      = &http_prefix,
+    [NXT_APP_RUBY]      = &http_prefix,
+    [NXT_APP_JAVA]      = &empty_prefix,
 };