use xzalloc in ada-lang.c:replace_operator_with_call

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.