Fix issue #488

Since I am new to the project and codebase, I am unsure if my fix is correct, but tested it using the method in issue #488 and it works. My concerns are however, as follows: why does this issue occur in PHP7, but not in PHP5?

Author: JoepRoebroek

src/CodeCoverage.php

@@ -370,6 +370,14 @@ public function append(array $data, $id = null, $append = true, $linesToBeCovere
                     if (empty($this->data[$file][$k]) || !in_array($id, $this->data[$file][$k])) {
                         $this->data[$file][$k][] = $id;
                     }
+                } else if ($v == Driver::LINE_NOT_EXECUTED) {
+                    if (!isset($this->data[$file][$k])) {
+                        $this->data[$file][$k] = [];
+                    }
+                } else if ($v == Driver::LINE_NOT_EXECUTABLE) {
+                    if (!isset($this->data[$file][$k])) {
+                        $this->data[$file][$k] = null;
+                    }
                 }
             }
         }