Fix jniThrowRuntimeException for C callers, add jniThrowNullPointerException.

...and switch all NPE throwers over to the helper.
diff --git a/libnativehelper/JNIHelp.c b/libnativehelper/JNIHelp.c
index d0e2f89..a75b837 100644
--- a/libnativehelper/JNIHelp.c
+++ b/libnativehelper/JNIHelp.c
@@ -133,6 +133,14 @@
 }
 
 /*
+ * Throw a java.lang.NullPointerException, with an optional message.
+ */
+int jniThrowNullPointerException(JNIEnv* env, const char* msg)
+{
+    return jniThrowException(env, "java/lang/NullPointerException", msg);
+}
+
+/*
  * Throw a java.lang.RuntimeException, with an optional message.
  */
 int jniThrowRuntimeException(JNIEnv* env, const char* msg)