jni: release string characters in conversion to Go string

This change should fix #13 by releasing the string buffer from the JVM after we
convert the string to a Go string. However, I've been having a lot of trouble
building the tests on Linux, and I've run out of time to keep messing with it.
I'm posting this patch in the hope that someone else is better positioned to
validate it. Please do not merge this unless you can verify that it works
properly.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

gojni.h

@@ -15,6 +15,7 @@ __attribute__ ((visibility ("hidden"))) jfieldID _jni_GetFieldID(JNIEnv *env, jc
 __attribute__ ((visibility ("hidden"))) jfieldID _jni_GetStaticFieldID(JNIEnv *env, jclass clazz, const char *name, const char *sig);
 __attribute__ ((visibility ("hidden"))) jsize _jni_GetStringLength(JNIEnv *env, jstring str);
 __attribute__ ((visibility ("hidden"))) const jchar *_jni_GetStringChars(JNIEnv *env, jstring str);
+__attribute__ ((visibility ("hidden"))) void _jni_ReleaseStringChars(JNIEnv *env, jstring str, const jchar *chars);
 __attribute__ ((visibility ("hidden"))) jstring _jni_NewString(JNIEnv *env, const jchar *unicodeChars, jsize len);
 __attribute__ ((visibility ("hidden"))) jboolean _jni_IsSameObject(JNIEnv *env, jobject ref1, jobject ref2);
 __attribute__ ((visibility ("hidden"))) jboolean _jni_IsInstanceOf(JNIEnv *env, jobject obj, jclass cls);

jni.c

@@ -78,6 +78,10 @@ const jchar *_jni_GetStringChars(JNIEnv *env, jstring str) {
 	return (*env)->GetStringChars(env, str, NULL);
 }
 
+void _jni_ReleaseStringChars(JNIEnv *env, jstring str, const jchar *chars) {
+	(*env)->ReleaseStringChars(env, str, chars);
+}
+
 jstring _jni_NewString(JNIEnv *env, const jchar *unicodeChars, jsize len) {
 	return (*env)->NewString(env, unicodeChars, len);
 }

jni.go

@@ -411,6 +411,10 @@ func GoString(e Env, str String) string {
 	}
 	strlen := C._jni_GetStringLength(e.env, C.jstring(str))
 	chars := C._jni_GetStringChars(e.env, C.jstring(str))
+	if chars == nil {
+		return ""
+	}
+	defer C._jni_ReleaseStringChars(e.env, C.jstring(str), chars)
 	var utf16Chars []uint16
 	hdr := (*reflect.SliceHeader)(unsafe.Pointer(&utf16Chars))
 	hdr.Data = uintptr(unsafe.Pointer(chars))