Fix two undefined-behavior issues

Fixes https://github.com/harfbuzz/harfbuzz/issues/761
Fixes https://github.com/harfbuzz/harfbuzz/issues/762

src/hb-ot-hmtx-table.hh

@@ -173,7 +173,7 @@ struct hmtxvmtx
 					 * be in the array, but that entry is
 					 * required. The last entry applies to
 					 * all subsequent glyphs. */
-  FWORD		leadingBearingX[VAR];	/* Here the advance is assumed
+/*FWORD		leadingBearingX[VAR];*/	/* Here the advance is assumed
 					 * to be the same as the advance
 					 * for the last entry above. The
 					 * number of entries in this array is
@@ -187,7 +187,7 @@ struct hmtxvmtx
 					 * font to vary the side bearing
 					 * values for each glyph. */
   public:
-  DEFINE_SIZE_ARRAY2 (0, longMetric, leadingBearingX);
+  DEFINE_SIZE_ARRAY (0, longMetric);
 };
 
 struct hmtx : hmtxvmtx<hmtx> {

src/hb-ot-layout-gsubgpos-private.hh

@@ -1212,7 +1212,7 @@ struct Rule
 		  lookupCount.sanitize (c) &&
 		  c->check_range (inputZ,
 				  inputZ[0].static_size * inputCount +
-				  lookupRecordX[0].static_size * lookupCount));
+				  LookupRecord::static_size * lookupCount));
   }
 
   protected:
@@ -1222,10 +1222,10 @@ struct Rule
   HBUINT16	lookupCount;		/* Number of LookupRecords */
   HBUINT16	inputZ[VAR];		/* Array of match inputs--start with
 					 * second glyph */
-  LookupRecord	lookupRecordX[VAR];	/* Array of LookupRecords--in
+/*LookupRecord	lookupRecordX[VAR];*/	/* Array of LookupRecords--in
 					 * design order */
   public:
-  DEFINE_SIZE_ARRAY2 (4, inputZ, lookupRecordX);
+  DEFINE_SIZE_ARRAY (4, inputZ);
 };
 
 struct RuleSet
@@ -1556,10 +1556,10 @@ struct ContextFormat3
   OffsetTo<Coverage>
 		coverageZ[VAR];		/* Array of offsets to Coverage
 					 * table in glyph sequence order */
-  LookupRecord	lookupRecordX[VAR];	/* Array of LookupRecords--in
+/*LookupRecord	lookupRecordX[VAR];*/	/* Array of LookupRecords--in
 					 * design order */
   public:
-  DEFINE_SIZE_ARRAY2 (6, coverageZ, lookupRecordX);
+  DEFINE_SIZE_ARRAY (6, coverageZ);
 };
 
 struct Context