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

Patchset: 1
http://review.cyanogenmod.org/#/c/65442/