[var] Fix getting side-bearing variations

In HVAR/VVAR, if the side-bearing mappings are null, it means the
table does not have them and they should be loaded from glyf table.
Previous logic was returning zer0.

Part of fixing https://github.com/harfbuzz/harfbuzz/issues/1694

src/hb-ot-hmtx-table.hh

@@ -241,8 +241,9 @@ struct hmtxvmtx
       if (unlikely (glyph >= num_bearings) || !font->num_coords)
 	return side_bearing;
 
-      if (var_table.get_length ())
-	return side_bearing + var_table->get_side_bearing_var (glyph, font->coords, font->num_coords);
+      float lsb;
+      if (var_table->get_side_bearing_var (glyph, font->coords, font->num_coords, &lsb))
+	return side_bearing + lsb;
 
       return _glyf_get_side_bearing_var (font, glyph, T::tableTag == HB_OT_TAG_vmtx);
 #else

src/hb-ot-var-hvar-table.hh

@@ -329,16 +329,16 @@ struct HVARVVAR
 				      store_cache);
   }
 
-  float get_side_bearing_var (hb_codepoint_t glyph,
-			      const int *coords, unsigned int coord_count) const
+  bool get_side_bearing_var (hb_codepoint_t glyph,
+			     const int *coords, unsigned int coord_count,
+			     float *lsb) const
   {
-    if (!has_side_bearing_deltas ()) return 0.f;
+    if (!lsbMap) return false;
     uint32_t varidx = (this+lsbMap).map (glyph);
-    return (this+varStore).get_delta (varidx, coords, coord_count);
+    *lsb = (this+varStore).get_delta (varidx, coords, coord_count);
+    return true;
   }
 
-  bool has_side_bearing_deltas () const { return lsbMap && rsbMap; }
-
   public:
   FixedVersion<>version;	/* Version of the metrics variation table
 				 * initially set to 0x00010000u */