[Coverage] Speed up subset for too-large Coverage tables

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=54980

src/OT/Layout/Common/Coverage.hh

@@ -147,6 +147,7 @@ struct Coverage
     TRACE_SUBSET (this);
     auto it =
     + iter ()
+    | hb_take (c->plan->source->get_num_glyphs ())
     | hb_filter (c->plan->glyph_map_gsub)
     | hb_map_retains_sorting (c->plan->glyph_map_gsub)
     ;