Correctly handle large headers in HPACK table

I missed a clause in section 4.4 of the HPACK spec when implementing
this stuff. Removes an errant assert and correctly handles adding large
entries to the HPACK table by flushing it and leaving it empty.

src/core/transport/chttp2/hpack_table.c

@@ -164,7 +164,21 @@ void grpc_chttp2_hptbl_add(grpc_chttp2_hptbl *tbl, grpc_mdelem *md) {
                           GRPC_CHTTP2_HPACK_ENTRY_OVERHEAD;
 
   /* we can't add elements bigger than the max table size */
-  assert(elem_bytes <= tbl->max_bytes);
+  if (elem_bytes > tbl->max_bytes) {
+    /* HPACK draft 10 section 4.4 states:
+     * If the size of the new entry is less than or equal to the maximum
+     * size, that entry is added to the table.  It is not an error to
+     * attempt to add an entry that is larger than the maximum size; an
+     * attempt to add an entry larger than the entire table causes
+     * the table
+     * to be emptied of all existing entries, and results in an
+     * empty table.
+     */
+    while (tbl->num_ents) {
+      evict1(tbl);
+    }
+    return;
+  }
 
   /* evict entries to ensure no overflow */
   while (elem_bytes > tbl->max_bytes - tbl->mem_used) {