Squiz/EmbeddedPhp: add some extra defensive coding

`$firstContentAfterBlock` should not be able to be `false` at this point as it is already checked and handled at the top of the function, but better to be safe than sorry.

src/Standards/Squiz/Sniffs/PHP/EmbeddedPhpSniff.php

@@ -284,7 +284,9 @@ private function validateMultilineEmbeddedPhp($phpcsFile, $stackPtr, $closingTag
                 $phpcsFile->fixer->addNewlineBefore($closingTag);
                 $phpcsFile->fixer->endChangeset();
             }//end if
-        } else if ($tokens[$firstContentAfterBlock]['line'] === $tokens[$closingTag]['line']) {
+        } else if ($firstContentAfterBlock !== false
+            && $tokens[$firstContentAfterBlock]['line'] === $tokens[$closingTag]['line']
+        ) {
             $error = 'Closing PHP tag must be on a line by itself';
             $fix   = $phpcsFile->addFixableError($error, $closingTag, 'ContentAfterEnd');
             if ($fix === true) {