[subset] fix fuzzer timeout.

Fixes: https://oss-fuzz.com/testcase-detail/6681253479579648. Limits iteration of coverage table during MATH subset to valid glyphs.

src/hb-iter.hh

@@ -842,7 +842,7 @@ struct
   template <typename Iterable,
 	    hb_requires (hb_is_iterable (Iterable))>
   auto operator () (Iterable&& it, unsigned count) const HB_AUTO_RETURN
-  ( hb_zip (hb_range (count), it) | hb_map (hb_second) )
+  ( hb_zip (hb_range (count), it) | hb_map_retains_sorting (hb_second) )
 
   /* Specialization arrays. */
 

src/hb-ot-math-table.hh

@@ -570,6 +570,7 @@ struct MathGlyphInfo
 
     auto it =
     + hb_iter (this+extendedShapeCoverage)
+    | hb_take (c->plan->source->get_num_glyphs ())
     | hb_filter (glyphset)
     | hb_map_retains_sorting (glyph_map)
     ;
@@ -941,13 +942,13 @@ struct MathVariants
     if (unlikely (!c->serializer->extend_min (out))) return_trace (false);
     if (!c->serializer->check_assign (out->minConnectorOverlap, minConnectorOverlap, HB_SERIALIZE_ERROR_INT_OVERFLOW))
       return_trace (false);
-    
+
     hb_sorted_vector_t<hb_codepoint_t> new_vert_coverage;
     hb_sorted_vector_t<hb_codepoint_t> new_hori_coverage;
     hb_set_t indices;
     collect_coverage_and_indices (new_vert_coverage, vertGlyphCoverage, 0, vertGlyphCount, indices, glyphset, glyph_map);
     collect_coverage_and_indices (new_hori_coverage, horizGlyphCoverage, vertGlyphCount, vertGlyphCount + horizGlyphCount, indices, glyphset, glyph_map);
-    
+
     if (!c->serializer->check_assign (out->vertGlyphCount, new_vert_coverage.length, HB_SERIALIZE_ERROR_INT_OVERFLOW))
       return_trace (false);
     if (!c->serializer->check_assign (out->horizGlyphCount, new_hori_coverage.length, HB_SERIALIZE_ERROR_INT_OVERFLOW))
@@ -959,10 +960,10 @@ struct MathVariants
       if (!o) return_trace (false);
       o->serialize_subset (c, glyphConstruction[i], this);
     }
-    
+
     if (new_vert_coverage)
       out->vertGlyphCoverage.serialize_serialize (c->serializer, new_vert_coverage.iter ());
-    
+
     if (new_hori_coverage)
     out->horizGlyphCoverage.serialize_serialize (c->serializer, new_hori_coverage.iter ());
     return_trace (true);