Only use offset optimization for string types in HEADERIMPORT_FAST (#398)

Only string types benefit from the offset-based optimization, for other
types the operation is a trivial constant-time calculation anyway and
getting inaccurate length information (due to alignment of next item
getting included) in return for a non-optimization is nothing but a
bad trade.

lib/header.c

@@ -497,7 +497,8 @@ static int regionSwab(indexEntry entry, int il, int dl,
 	if (dataEnd && (unsigned char *)ie.data >= dataEnd)
 	    return -1;
 
-	if (fast && il > 1) {
+	/* The offset optimization is only relevant for string types */
+	if (fast && il > 1 && typeSizes[ie.info.type] == -1) {
 	    ie.length = ntohl(pe[1].offset) - ie.info.offset;
 	} else {
 	    ie.length = dataLength(ie.info.type, ie.data, ie.info.count,