Fix compile error with Android NDK and --enable-werror

lib/nghttp2_hd.c

@@ -1321,7 +1321,8 @@ static search_result search_hd_table(nghttp2_hd_context *context,
     return res;
   }
 
-  res.index = context->next_seq - 1 - ent->seq + NGHTTP2_STATIC_TABLE_LENGTH;
+  res.index =
+      (ssize_t)(context->next_seq - 1 - ent->seq + NGHTTP2_STATIC_TABLE_LENGTH);
 
   if (exact_match) {
     res.name_value_match = 1;

lib/nghttp2_hd_huffman.c

@@ -157,7 +157,8 @@ int nghttp2_hd_huff_encode(nghttp2_bufs *bufs, const uint8_t *src,
     const nghttp2_huff_sym *sym = &huff_sym_table[256];
     assert(avail);
     /* Caution we no longer adjust avail here */
-    nghttp2_bufs_fast_orb(bufs, (uint8_t)(sym->code >> (sym->nbits - rembits)));
+    nghttp2_bufs_fast_orb(
+        bufs, (uint8_t)(sym->code >> (sym->nbits - (size_t)rembits)));
   }
 
   return 0;