Free ast_str objects when temp file fails to be created in MiniVM

The previous commit (r372554) was from a patch that was written before
r366880, which ensured that ast_str objects allocated in the sendmail
routine were free'd in off nominal paths.  This commit frees the
string objects in the off nominal path introduced in r372554.

(issue ASTERISK-17133)
Reported by: Tzafrir Cohen
........

Merged revisions 372581 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 372582 from http://svn.asterisk.org/svn/asterisk/branches/10
........

Merged revisions 372583 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@372584 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_minivm.c

@@ -1277,6 +1277,8 @@ static int sendmail(struct minivm_template *template, struct minivm_account *vmu
 		tmpfd = mkstemp(newtmp);
 		if (tmpfd < 0) {
 			ast_log(LOG_WARNING, "Failed to create temporary file for volgain: %d\n", errno);
+			ast_free(str1);
+			ast_free(str2);
 			return -1;
 		}
 		snprintf(tmpcmd, sizeof(tmpcmd), "sox -v %.4f %s.%s %s.%s", vmu->volgain, filename, format, newtmp, format);