fix: close only existing upvalues for locals in scope during stack unwinding

The previous implementation unconditionally closed the first open upvalue when
executing CLOSE_UPVALUE, which could crash when a local variable was never
actually captured by a closure. Changed closeUpvalue to closeUpvalues with a
stack threshold parameter, so only upvalues whose stack slot is at or above the
given pointer are closed. This prevents closing nonexistent upvalues for locals
that were never closed over, fixing segfaults in unused closure scenarios.

Added regression tests for unused closures and for nested scopes where only
some locals are captured.

src/vm/wren_vm.c

@@ -260,16 +260,22 @@ static ObjUpvalue* captureUpvalue(WrenVM* vm, ObjFiber* fiber, Value* local)
   return createdUpvalue;
 }
 
-static void closeUpvalue(ObjFiber* fiber)
+// Closes any open upvates that have been created for stack slots at [last] and
+// above.
+static void closeUpvalues(ObjFiber* fiber, Value* last)
 {
-  ObjUpvalue* upvalue = fiber->openUpvalues;
-
-  // Move the value into the upvalue itself and point the upvalue to it.
-  upvalue->closed = *upvalue->value;
-  upvalue->value = &upvalue->closed;
-
-  // Remove it from the open upvalue list.
-  fiber->openUpvalues = upvalue->next;
+  while (fiber->openUpvalues != NULL &&
+         fiber->openUpvalues->value >= last)
+  {
+    ObjUpvalue* upvalue = fiber->openUpvalues;
+    
+    // Move the value into the upvalue itself and point the upvalue to it.
+    upvalue->closed = *upvalue->value;
+    upvalue->value = &upvalue->closed;
+    
+    // Remove it from the open upvalue list.
+    fiber->openUpvalues = upvalue->next;
+  }
 }
 
 // Looks up a foreign method in [moduleName] on [className] with [signature].
@@ -1079,7 +1085,8 @@ static WrenInterpretResult runInterpreter(WrenVM* vm, register ObjFiber* fiber)
     }
 
     CASE_CODE(CLOSE_UPVALUE):
-      closeUpvalue(fiber);
+      // Close the upvalue for the local if we have one.
+      closeUpvalues(fiber, fiber->stackTop - 1);
       DROP();
       DISPATCH();
 
@@ -1089,13 +1096,8 @@ static WrenInterpretResult runInterpreter(WrenVM* vm, register ObjFiber* fiber)
       fiber->numFrames--;
 
       // Close any upvalues still in scope.
-      Value* firstValue = stackStart;
-      while (fiber->openUpvalues != NULL &&
-             fiber->openUpvalues->value >= firstValue)
-      {
-        closeUpvalue(fiber);
-      }
-
+      closeUpvalues(fiber, stackStart);
+      
       // If the fiber is complete, end it.
       if (fiber->numFrames == 0)
       {