Prevent some E_NOTICE in boot.php

Signed-off-by: Roland Haeder <roland@mxchange.org>

boot.php

@@ -1047,11 +1047,21 @@ class App {
 	function save_timestamp($stamp, $value) {
 		$duration = (float)(microtime(true)-$stamp);
 
+		if (!isset($this->performance[$value])) {
+			// Prevent ugly E_NOTICE
+			$this->performance[$value] = 0;
+		}
+
 		$this->performance[$value] += (float)$duration;
 		$this->performance["marktime"] += (float)$duration;
 
 		$callstack = $this->callstack();
 
+		if (!isset($this->callstack[$value][$callstack])) {
+			// Prevent ugly E_NOTICE
+			$this->callstack[$value][$callstack] = 0;
+		}
+
 		$this->callstack[$value][$callstack] += (float)$duration;
 
 	}