Rust: Call refactor follow-up fixes

Author: hvitved

rust/ql/lib/codeql/rust/elements/internal/CallExprImpl.qll

@@ -95,7 +95,7 @@ module Impl {
    * layer, we do not check that the resolved target is a method in the charpred,
    * instead we check this in `getPositionalArgument` and `getReceiver`.
    */
-  class CallExprMethodCall extends CallImpl::MethodCall instanceof CallExprCall {
+  class CallExprMethodCall extends CallImpl::MethodCall, CallExprCall {
     CallExprMethodCall() { not this instanceof DynamicCallExpr }
 
     private predicate isInFactMethodCall() { this.getResolvedTarget() instanceof Method }

rust/ql/lib/codeql/rust/elements/internal/OperationImpl.qll

@@ -140,7 +140,7 @@ module Impl {
   private class OperationMethodCall extends CallImpl::MethodCall instanceof Operation {
     OperationMethodCall() { super.isOverloaded(_, _, _) }
 
-    override Expr getPositionalArgument(int i) { result = super.getOperand(i + 1) }
+    override Expr getPositionalArgument(int i) { result = super.getOperand(i + 1) and i >= 0 }
 
     override Expr getReceiver() { result = super.getOperand(0) }
   }