bind/java: pin java byte array elements until Seq send is done.

When passing a byte array from Java to Go, Seq.writeByteArray JNI
call encodes only the array size and the pointer to the array.
Go-side receives the (size, ptr) pair info during the subsequent
Seq.send JNI call, and copies the elements into a Go byte slice.
We must pin the array elements until Go-side completes copying
so that they are not moved or collected by Java runtime.

This change keeps track of the pinned array info in a 'pinned' linked
list, and unpin them as the Seq memory is freed.  The jbyteArray
argument passed to Seq.writeByteArray is needed to release the pinned
byte array elements, but that is a "local reference". It is not
guaranteed that the reference is valid after the method returns. Thus,
we stash its global reference in the 'pinned' list and delete it later
as well.

A similar problem can occur on the byte slice returned from a Go function.
This change does not address the case yet.

Fixes golang/go#9486

Change-Id: I1255aefbc80b21ccbe9b2bf37699faaf0c5f0bae
Reviewed-on: https://go-review.googlesource.com/2586
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/java/SeqTest.java

@@ -81,6 +81,27 @@ public class SeqTest extends TestCase {
     }
   }
 
+  // Test for golang.org/issue/9486.
+  public void testByteArrayAfterString() {
+    byte[] bytes = new byte[1024];
+    for (int i=0; i < bytes.length; i++) {
+           bytes[i] = 8;
+    }
+
+    String stuff = "stuff";
+    byte[] got = Testpkg.AppendToString(stuff, bytes);
+
+    try {
+      byte[] s = stuff.getBytes("UTF-8");
+      byte[] want = new byte[s.length + bytes.length];
+      System.arraycopy(s, 0, want, 0, s.length);
+      System.arraycopy(bytes, 0, want, s.length, bytes.length);
+      MoreAsserts.assertEquals("Bytes should match", want, got);
+    } catch (Exception e) {
+      fail("Cannot perform the test: " + e.toString());
+    }
+  }
+
   public void testGoRefGC() {
     Testpkg.S s = Testpkg.New();
     runGC();