use xzalloc in ada-lang.c:replace_operator_with_call

This is to make sure that we cannot have a field with an undefined
value (currently, we are not setting the gdbarch).

gdb/ChangeLog:

	* ada-lang.c (replace_operator_with_call): Use xzalloc instead
	of xmalloc.

gdb/ada-lang.c

@@ -3655,7 +3655,7 @@ replace_operator_with_call (struct expression **expp, int pc, int nargs,
   /* A new expression, with 6 more elements (3 for funcall, 4 for function
      symbol, -oplen for operator being replaced).  */
   struct expression *newexp = (struct expression *)
-    xmalloc (sizeof (struct expression)
+    xzalloc (sizeof (struct expression)
              + EXP_ELEM_TO_BYTES ((*expp)->nelts + 7 - oplen));
   struct expression *exp = *expp;