jni: call `ReleaseStringUTFChars` where appropriate

According to [this resource](http://journals.ecs.soton.ac.uk/java/tutorial/native1.1/implementing/string.html):

> Note that when your native code is finished using the UTF-8 string, it must call ReleaseStringUTFChars. ReleaseStringUTFChars informs the VM that the native method is finished with the string, so that the memory taken by the UTF-8 string can then be freed. Failing to call ReleaseStringUTFChars results in a memory leak. This will ultimately lead to system memory exhaustion.

Unfortunately, there are many places in jni_interface.cc where this does not happen.