Fix crash when foreach key isn't a variable

[phil-nelson]

This fixes a crash when a foreach key is not a straight up variable (which I hadn't realised was possible), e.g. this is valid:

<?php $arr = [1,2,3]; $a = new stdClass(); foreach ($arr as $a->key => $value) { echo "{$a->key}: $value\n"; }

Fixes #563

[codecov]

Codecov Report

Merging #564 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## master #564 +/- ## ============================================ + Coverage 81.46% 81.52% +0.05%  Complexity 901 901 ============================================ Files 61 61 Lines 2061 2062 +1 ============================================ + Hits 1679 1681 +2  + Misses 382 381 -1

Impacted Files	Coverage Δ	Complexity Δ
src/DefinitionResolver.php	87.03% <100%> (+0.02%)	321 <0> (ø)	⬇️
src/Cache/FileSystemCache.php	25% <0%> (+5%)	8% <0%> (ø)	⬇️