Fix bug in ResponseWriter for variadic MaD

`getSummaryInputOrOutputNode` was giving the summary component stack for
arguments corresponding to variadic parameters. This will be a problem
when the models for variadic functions are converted to models-as-data.

Author: owen-mc

go/ql/lib/semmle/go/frameworks/stdlib/NetHttp.qll

@@ -123,9 +123,19 @@ module NetHttp {
   private DataFlow::Node getSummaryInputOrOutputNode(
     DataFlow::CallNode call, SummaryComponentStack stack
   ) {
-    exists(int n |
-      stack = SummaryComponentStack::argument(n) and
-      result = call.getArgument(n)
+    exists(int n | result = call.getSyntacticArgument(n) |
+      if result = call.getImplicitVarargsArgument(_)
+      then
+        exists(
+          int lastParamIndex, SummaryComponentStack varArgsSliceArgument,
+          SummaryComponent arrayContentSC, DataFlow::ArrayContent arrayContent
+        |
+          lastParamIndex = call.getCall().getCalleeType().getNumParameter() - 1 and
+          varArgsSliceArgument = SummaryComponentStack::argument(lastParamIndex) and
+          arrayContentSC = SummaryComponent::content(arrayContent) and
+          stack = SummaryComponentStack::push(arrayContentSC, varArgsSliceArgument)
+        )
+      else stack = SummaryComponentStack::argument(n)
     )
     or
     stack = SummaryComponentStack::argument(-1) and