clang compiler warnings: Fix sometimes-initialized warning in func_math

This patch fixes a bug in a unit test in func_math where a variable could be
passed to ast_free that wasn't allocated. This patch corrects the issue and
ensures that we only attempt to free a variable if we previously allocated
it.

Review: https://reviewboard.asterisk.org/r/4552

ASTERISK-24917
Reported by: dkdegroot
patches:
  rb4552.patch submitted by dkdegroot (License 6600)
........

Merged revisions 434190 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@434191 65c4cc65-6c06-0410-ace0-fbb531ad65f3

funcs/func_math.c

@@ -482,13 +482,11 @@ AST_TEST_DEFINE(test_MATH_function)
 
 	ast_test_status_update(test, "Testing MATH() substitution ...\n");
 
-	if (!(expr = ast_str_create(16)) || !(result = ast_str_create(16))) {
-		if (expr) {
-			ast_free(expr);
-		}
-		if (result) {
-			ast_free(result);
-		}
+	if (!(expr = ast_str_create(16))) {
+		return AST_TEST_FAIL;
+	}
+	if (!(result = ast_str_create(16))) {
+		ast_free(expr);
 		return AST_TEST_FAIL;
 	}