ui/app: (android) fix length of C string copy

The data dir string is returned to Go as a byte array of the UTF-8
encoded string, but it is not NUL terminated.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/datadir_android.go

@@ -9,8 +9,8 @@ import "C"
 var dataDir func() (string, error)
 
 //export setDataDir
-func setDataDir(cdir *C.char) {
-	dir := C.GoString(cdir)
+func setDataDir(cdir *C.char, len C.int) {
+	dir := C.GoStringN(cdir, len)
 	dataDir = func() (string, error) {
 		return dir, nil
 	}

ui/app/os_android.c

@@ -106,7 +106,8 @@ JNIEXPORT jint JNI_OnLoad(JavaVM *vm, void *reserved) {
 	if (dir == NULL) {
 		return -1;
 	}
-	setDataDir((char *)dir);
+	jint n = (*env)->GetArrayLength(env, dirArr);
+	setDataDir((char *)dir, n);
 	(*env)->ReleaseByteArrayElements(env, dirArr, dir, JNI_ABORT);
 
 	return JNI_VERSION_1_6;