[Yaml] throw a ParseException on invalid data type

Without this check, PHP would trigger a warning when an array was passed
to `trim()`. The parser must throw a `ParseException` instance on a
malformed YAML string instead.

src/Symfony/Component/Yaml/Parser.php

@@ -234,7 +234,7 @@ class Parser
                 }
 
                 // 1-liner optionally followed by newline(s)
-                if ($this->lines[0] === trim($value)) {
+                if (is_string($value) && $this->lines[0] === trim($value)) {
                     try {
                         $value = Inline::parse($this->lines[0], $exceptionOnInvalidType, $objectSupport, $objectForMap, $this->refs);
                     } catch (ParseException $e) {

src/Symfony/Component/Yaml/Tests/ParserTest.php

@@ -551,6 +551,21 @@ EOF
         );
     }
 
+    /**
+     * @expectedException \Symfony\Component\Yaml\Exception\ParseException
+     * @expectedExceptionMessage missing colon
+     */
+    public function testScalarInSequence()
+    {
+        Yaml::parse(<<<EOF
+foo:
+    - bar
+"missing colon"
+    foo: bar
+EOF
+        );
+    }
+
     /**
      * > It is an error for two equal keys to appear in the same mapping node.
      * > In such a case the YAML processor may continue, ignoring the second