MtpDatabase JNI: Fixed a memory-leak in getObjectPropertyValue().

The jstring "stringValue" was not never freed.
In the case where "str" was NULL the whole cleanup part (see "goto out")
was even skipped.
This patch makes getObjectPropertyValue() behave like
getObjectPropertyList().

Change-Id: I5a7ec3611036f5253a054b00064999bcd1d1c29e