http2: rename counter in mapToHeaders inner loop

This change is to prevent potential bugs - e.g., someone might
automatically use the variable `k` instead of `key`, that is used in
vicinity of this loop.
Also changed postincrement to preincrement in iteration steps. It is
probably done by the optimizer anyway, but otherwise it will save an
opcode each iteration. And it is a good practice.

PR-URL: #32012
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/internal/http2/util.js

@@ -442,13 +442,13 @@ function mapToHeaders(map,
   let count = 0;
   const keys = ObjectKeys(map);
   const singles = new Set();
-  let i;
+  let i, j;
   let isArray;
   let key;
   let value;
   let isSingleValueHeader;
   let err;
-  for (i = 0; i < keys.length; i++) {
+  for (i = 0; i < keys.length; ++i) {
     key = keys[i];
     value = map[key];
     if (value === undefined || key === '')
@@ -488,8 +488,8 @@ function mapToHeaders(map,
       throw new ERR_HTTP2_INVALID_CONNECTION_HEADERS(key);
     }
     if (isArray) {
-      for (var k = 0; k < value.length; k++) {
-        const val = String(value[k]);
+      for (j = 0; j < value.length; ++j) {
+        const val = String(value[j]);
         ret += `${key}\0${val}\0`;
       }
       count += value.length;