Add validation to Branch name before switching

Related to https://www.huntr.dev/bounties/29-other-FalconChristmas/fpp/

scripts/git_branch

@@ -15,6 +15,12 @@ teeOutput
 
 echo "$0 running to switch branches to $1"
 
+CNT=`cd ${FPPDIR} && git show-ref | grep "refs/remotes/origin" | grep -i $1 | wc -l`
+if [ $CNT -eq 0 ]; then
+	echo "Invalid Branch Name: $1"
+	exit 1
+fi
+
 echo "Stopping fppd"
 ${SUDO} ${FPPDIR}/scripts/fppd_stop
 

www/changebranch.php

@@ -19,7 +19,7 @@
 <?php
     echo "==================================================================================\n";
 
-    $branch = escapeshellcmd($_GET['branch']);
+    $branch = escapeshellcmd(htmlspecialchars($_GET['branch']));
 	$command = "sudo /opt/fpp/scripts/git_branch " . $branch . " 2>&1";
 
 	echo "Command: $command\n";