app_voicemail: Fix ill-formatted pager emails with custom subject.

Add missing end-of-headers newline to pager emails with custom
subjects, since this was missing from this code path.

Resolves: #902
(cherry picked from commit e63da10a17)

apps/app_voicemail.c

@@ -6033,12 +6033,15 @@ static int sendpage(char *srcemail, char *pager, int msgnum, char *context, char
 		}
 	} else {
 		if (ast_strlen_zero(flag)) {
-			fprintf(p, "Subject: New VM\n\n");
+			fprintf(p, "Subject: New VM" ENDL);
 		} else {
-			fprintf(p, "Subject: New %s VM\n\n", flag);
+			fprintf(p, "Subject: New %s VM" ENDL, flag);
 		}
 	}
 
+	/* End of headers */
+	fputs(ENDL, p);
+
 	if (pagerbody) {
 		struct ast_channel *ast;
 		if ((ast = ast_dummy_channel_alloc())) {