app_voicemail: Add workaround for a gcc 10 issue with -Wrestrict

The gcc 10 -Wrestrict option was causing "overlap" errors when
snprintf was copying one char[256] structure member to another
char[256] member in the same structure.

Using ast_alloca instead of declaring the structure inline
solves the issue.

Here's a link to the "enhancement":
https://gcc.gnu.org/legacy-ml/gcc-patches/2019-10/msg00570.html

We may follow up with a gcc bug report.

Change-Id: Ie0099adcb0a9727bd9aa99e024dd912a67eaf534

apps/app_voicemail.c

@@ -14662,6 +14662,12 @@ exit_vmsayname_test:
 	return res ? AST_TEST_FAIL : AST_TEST_PASS;
 }
 
+struct test_files {
+	char dir[256];
+	char file[256];
+	char txtfile[256];
+};
+
 AST_TEST_DEFINE(test_voicemail_msgcount)
 {
 	int i, j, res = AST_TEST_PASS, syserr;
@@ -14671,11 +14677,8 @@ AST_TEST_DEFINE(test_voicemail_msgcount)
 #ifdef IMAP_STORAGE
 	struct ast_channel *chan = NULL;
 #endif
-	struct {
-		char dir[256];
-		char file[256];
-		char txtfile[256];
-	} tmp[3];
+	/* Using ast_alloca instead of just declaring tmp as an array is a workaround for a GCC 10 issue with -Wrestrict */
+	struct test_files *tmp = ast_alloca(sizeof(struct test_files) * 3);
 	char syscmd[256];
 	const char origweasels[] = "tt-weasels";
 	const char testcontext[] = "test";