fw: media: Handle NPE due to threading race

Change-Id: I6eab5bc3d687edfa6f0839e439b3314bed4fdcea
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