Context: dotnet/android#7285 (comment) (#1029)

`Java.Inteorp.JniEnvironment.Types.RegisterNatives()` is overloaded:
namespace Java.Interop {
partial class JniEnvironment {
partial class Types {
public static void RegisterNatives (JniObjectReference type, JniNativeMethodRegistration[] methods);
public static void RegisterNatives (JniObjectReference type, JniNativeMethodRegistration[] methods, int numMethods);
}
}
}
If the `int numMethods` overload is used, then:
1. it should be possible to pass an array which contains *more* than
`numMethods` elements, and
2. Passing such an array shouldn't throw an exception.
For example:
var methods = new JniNativeMethodRegistration [10];
JniEnvironment.Types.RegisterNatives (type, methods, 0);
Instead, when using a Debug configuration build of `Java.Interop.dll`,
a `NullReferenceException` would be thrown, because the `foreach`
loop would traverse *every element*, not just the first `numMethods`
elements, which could result in accessing
`methods[i].Marshaler.GetType()`, which could be `null`.
Fix the `DEBUG && NETCOREAPP` check so that only the first
`numMethods` elements are accessed, *not* all of them, and add a
`null` check around `JniNativeMethodRegistration.Marshaler` access.
This prevents the `NullReferenceException`.
Additionally, add some parameter validation and throw an
`ArgumentOutOfRangeException` if `numMethods` is negative or is
greater than `methods.Length`.

Author: jonpryor

src/Java.Interop/Java.Interop/JniEnvironment.Types.cs

@@ -200,9 +200,15 @@ public static void RegisterNatives (JniObjectReference type, JniNativeMethodRegi
 
 publicstaticvoidRegisterNatives(JniObjectReferencetype,JniNativeMethodRegistration[]methods,intnumMethods)
 {
+if((numMethods<0)||
+(numMethods>(methods?.Length??0))){
+thrownewArgumentOutOfRangeException(nameof(numMethods),numMethods,
+$"`numMethods` must be between 0 and `methods.Length` ({methods?.Length??0})!");
+}
 #if DEBUG&&NETCOREAPP
-foreach(varminmethods){
-if(m.Marshaler.GetType().GenericTypeArguments.Length!=0){
+for(inti=0;methods!=null&&i<numMethods;++i){
+varm=methods[i];
+if(m.Marshaler!=null&&m.Marshaler.GetType().GenericTypeArguments.Length!=0){
 varmethod=m.Marshaler.Method;
 Debug.WriteLine($"JNIEnv::RegisterNatives() given a generic delegate type `{m.Marshaler.GetType()}`. .NET Core doesn't like this.");
 Debug.WriteLine($" Java: {m.Name}{m.Signature}");