res_fax: Don't consume frames given to fax gateway on write.

In a particular fax gateway scenario whereby it would
have to translate using the read translation path on a
channel the frame being translated would be consumed.
When the frame is in the write path it is not permitted
to free the frame as the caller expects it to continue
to exist.

This change makes it so that the frame is only consumed
on the read path where it is acceptable to free it.

ASTERISK-28900

Change-Id: I011c321288a1b056d92b37c85e229f4a28ee737d

res/res_fax.c

@@ -3568,7 +3568,7 @@ static struct ast_frame *fax_gateway_framehook(struct ast_channel *chan, struct
 		 * translation is done, so we need to translate here */
 		if ((f->frametype == AST_FRAME_VOICE) && (ast_format_cmp(f->subclass.format, ast_format_slin) != AST_FORMAT_CMP_EQUAL)
 			&& (readtrans = ast_channel_readtrans(active))) {
-			if ((f = ast_translate(readtrans, f, 1)) == NULL) {
+			if ((f = ast_translate(readtrans, f, event == AST_FRAMEHOOK_EVENT_WRITE ? 0 : 1)) == NULL) {
 				f = &ast_null_frame;
 				return f;
 			}