Only make a best-effort to release mStacks

It doesn't *actually* matter, and getting rid of the defer is worth
~80ns of perf.

web/mux.go

@@ -74,16 +74,14 @@ func New() *Mux {
 
 func (m *Mux) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	stack := m.mStack.alloc()
-	defer m.mStack.release(stack)
-
 	stack.ServeHTTP(w, r)
+	m.mStack.release(stack)
 }
 
 // ServeHTTPC creates a context dependent request with the given Mux. Satisfies
 // the web.Handler interface.
 func (m *Mux) ServeHTTPC(c C, w http.ResponseWriter, r *http.Request) {
 	stack := m.mStack.alloc()
-	defer m.mStack.release(stack)
-
 	stack.ServeHTTPC(c, w, r)
+	m.mStack.release(stack)
 }