FIX IntrospectionProcessor: E_NOTICE

Recent merge of #608 misses check whether the trace exists at all at the specific index, leading to undefined offset.

```
E_NOTICE: Undefined offset: 3
```

And because the while statement became unreadable (and too long), I moved it into a separate method.

src/Monolog/Processor/IntrospectionProcessor.php

@@ -61,7 +61,7 @@ class IntrospectionProcessor
 
         $i = 0;
 
-        while (isset($trace[$i]['class']) || in_array($trace[$i]['function'], $this->skipFunctions)) {
+        while ($this->isTraceClassOrNotSkippedFunction($trace, $i)) {
             if (isset($trace[$i]['class'])) {
                 foreach ($this->skipClassesPartials as $part) {
                     if (strpos($trace[$i]['class'], $part) !== false) {
@@ -90,4 +90,13 @@ class IntrospectionProcessor
 
         return $record;
     }
+
+    private function isTraceClassOrNotSkippedFunction ($trace, $index)
+    {
+        if (isset($trace[$index]) === false) {
+            return false;
+        }
+
+        return isset($trace[$index]['class']) || in_array($trace[$index]['function'], $this->skipFunctions);
+    }
 }