Don't pop when setting an upvalue.

Fix munificent#40.

Since assignment is an expression, every assignment instruction
(SET_LOCAL, SET_GLOBAL, SET_UPVALUE, SET_PROPERTY) needs to leave the
RHS on the stack. SET_UPVALUE was accidentally consuming it, which
leaves the stack in the wrong state.

c/compiler.c

@@ -371,7 +371,8 @@ static ObjFunction* endCompiler() {
     disassembleChunk(currentChunk(), "code");
 */
 //> Calls and Functions not-yet
-    disassembleChunk(currentChunk(), function->name->chars);
+    disassembleChunk(currentChunk(),
+        function->name != NULL ? function->name->chars : "<top>");
 //< Calls and Functions not-yet
   }
 #endif

c/vm.c

@@ -550,7 +550,7 @@ static bool run() {
 
       case OP_SET_UPVALUE: {
         uint8_t slot = READ_BYTE();
-        *frame->closure->upvalues[slot]->value = pop();
+        *frame->closure->upvalues[slot]->value = peek(0);
         break;
       }
 //< Closures not-yet

test/regression/40.lox

@@ -0,0 +1,22 @@
+fun caller(g) {
+  g();
+  // g should be a function, not nil.
+  print g == nil; // expect: false
+}
+
+fun callCaller() {
+  var capturedVar = "before";
+  var a = "a";
+
+  fun f() {
+    // Commenting the next line out prevents the bug!
+    capturedVar = "after";
+
+    // Returning anything also fixes it, even nil:
+    //return nil;
+  }
+
+  caller(f);
+}
+
+callCaller();