In xml_convert, handle the case when repl is the empty string on the

first iteration. This can't happen with the current data definitions,
but if it did it would result in memcpy being passed a null pointer.
(memcpy would be asked to do a zero-byte copy, so it would probably be
okay anyway, but it's better to be safe.)

output.cc

@@ -912,7 +912,7 @@ char *xml_convert(const char *str) {
 
     len = strlen(repl);
     /* Double the size of the result buffer if necessary. */
-    if (i + len > n) {
+    if (i == 0 || i + len > n) {
       n = (i + len) * 2;
       result = (char *) safe_realloc(result, n + 1);
     }