Prettify JNI stuff.

native/src/main/c/jni_zlib_common.c

@@ -0,0 +1,29 @@
+#include <jni.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <zlib.h>
+#include "jni_util.h"
+
+void JNICALL
+check_zlib_free(JNIEnv *env, z_stream *stream, bool deflate)
+{
+    int ret = deflate ? deflateEnd(stream) : inflateEnd(stream);
+    char *msg = stream->msg;
+    free((void*) stream);
+
+    switch (ret) {
+        case Z_OK:
+            break;
+        case Z_STREAM_ERROR:
+            if (msg == NULL) {
+                msg = "stream state inconsistent";
+            }
+            // fall-through
+        case Z_DATA_ERROR:
+            if (msg == NULL) {
+                msg = "data was discarded";
+            }
+            throwException(env, "java/lang/IllegalArgumentException", msg);
+            break;
+    }
+}