nghttp2_hd_deflate_bound: Take into account possible 2nd context update

lib/nghttp2_hd.c

@@ -1136,8 +1136,9 @@ size_t nghttp2_hd_deflate_bound(nghttp2_hd_deflater *deflater,
   size_t i;
 
   /* Possible Maximum Header Table Size Change.  Encoding (1u << 31) -
-     1 using 4 bit prefix requires 6 bytes. */
-  n += 6;
+     1 using 4 bit prefix requires 6 bytes.  We may emit this at most
+     twice. */
+  n += 12;
 
   /* Use Literal Header Field without indexing - New Name, since it is
      most space consuming format.  Also we choose the less one between

tests/nghttp2_hd_test.c

@@ -1081,7 +1081,7 @@ void test_nghttp2_hd_deflate_bound(void)
 
   bound = nghttp2_hd_deflate_bound(&deflater, nva, ARRLEN(nva));
 
-  CU_ASSERT(6 + 6 * 2 * 2 +
+  CU_ASSERT(12 + 6 * 2 * 2 +
             nva[0].namelen + nva[0].valuelen +
             nva[1].namelen + nva[1].valuelen
             == bound);