Certain methods in imageioJPEG.c (the glue code between the IJG native JPEG
libraries and the Image I/O API) such as readImageHeader(), readImage(),
writeImageHeader(), and writeImage() make JNI calls in the scope of
Get/ReleasePrimitiveArrayCritical. Most Java upcalls in the file have been
correctly surrounded with RELEASE/GET_ARRAYS macros, but a few cases fell
through the cracks.

Running any application that reads or writes JPEG images with the non-standard
-Xcheck:jni flag causes the following error (or something similar):

FATAL ERROR in native method: Calling other JNI functions in the scope of Get/ReleasePrimitiveArrayCritical or Get/ReleaseStringCritical
at com.sun.imageio.plugins.jpeg.JPEGImageReader.readImageHeader(Native Method)
at com.sun.imageio.plugins.jpeg.JPEGImageReader.readNativeHeader(JPEGImageReader.java:545)
at com.sun.imageio.plugins.jpeg.JPEGImageReader.checkTablesOnly(JPEGImageReader.java:300)
at com.sun.imageio.plugins.jpeg.JPEGImageReader.gotoImage(JPEGImageReader.java:422)
at com.sun.imageio.plugins.jpeg.JPEGImageReader.readHeader(JPEGImageReader.java:538)
at com.sun.imageio.plugins.jpeg.JPEGImageReader.readInternal(JPEGImageReader.java:871)
at com.sun.imageio.plugins.jpeg.JPEGImageReader.read(JPEGImageReader.java:855)
at javax.imageio.ImageIO.read(ImageIO.java:1317)
at javax.imageio.ImageIO.read(ImageIO.java:1241)
at Test.<init>(Test.java:29)
at Test.main(Test.java:40)

These problem areas are unlikely to cause problems, but we should try to
conform to the JNI specs, which mandate that no JNI upcalls be made inside
of a critical section.
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