JIT: fix assert seen in some OSR cases (dotnet#68048)

As a result of dotnet#67884, OSR compilations were looking at data in the
`gsShadowVarInfo` array in cases where it was not initialized.

Fix is to null out `gsShadowVarInfo` array if there are no shadowed params.

Fixes dotnet#68003.

src/coreclr/jit/gschecks.cpp

@@ -53,16 +53,26 @@ void Compiler::gsCopyShadowParams()
     }
 
     // Allocate array for shadow param info
+    //
     gsShadowVarInfo = new (this, CMK_Unknown) ShadowParamVarInfo[lvaCount]();
 
     // Find groups of variables assigned to each other, and also
     // tracks variables which are dereferenced and marks them as ptrs.
     // Look for assignments to *p, and ptrs passed to functions
+    //
     if (gsFindVulnerableParams())
     {
         // Replace vulnerable params by shadow copies.
+        //
         gsParamsToShadows();
     }
+    else
+    {
+        // There are no vulnerable params.
+        // Clear out the info to avoid looking at stale data.
+        //
+        gsShadowVarInfo = nullptr;
+    }
 }
 
 // This struct tracks how a tree is being used