[core] extract weak_ptr mailbox->receive into Mailbox::maybeReceive

include/mbgl/util/run_loop.hpp

@@ -83,9 +83,7 @@ private:
 
     void schedule(std::weak_ptr<Mailbox> mailbox) override {
         invoke([mailbox] () {
-            if (auto locked = mailbox.lock()) {
-                locked->receive();
-            }
+            Mailbox::maybeReceive(mailbox);
         });
     }
 

platform/default/thread_pool.cpp

@@ -22,9 +22,7 @@ ThreadPool::ThreadPool(std::size_t count) {
                 queue.pop();
                 lock.unlock();
 
-                if (auto locked = mailbox.lock()) {
-                    locked->receive();
-                }
+                Mailbox::maybeReceive(mailbox);
             }
         });
     }

platform/node/src/node_thread_pool.cpp

@@ -27,9 +27,7 @@ NodeThreadPool::Worker::Worker(std::weak_ptr<mbgl::Mailbox> mailbox_)
     mailbox(std::move(mailbox_)) {};
 
 void NodeThreadPool::Worker::Execute() {
-    if (auto locked = mailbox.lock()) {
-        locked->receive();
-    }
+    mbgl::Mailbox::maybeReceive(mailbox);
 }
 
 void NodeThreadPool::Worker::WorkComplete() {

src/mbgl/actor/mailbox.cpp

@@ -52,4 +52,10 @@ void Mailbox::receive() {
     }
 }
 
+void Mailbox::maybeReceive(std::weak_ptr<Mailbox> mailbox) {
+    if (auto locked = mailbox.lock()) {
+        locked->receive();
+    }
+}
+
 } // namespace mbgl

src/mbgl/actor/mailbox.hpp

@@ -18,6 +18,8 @@ public:
     void close();
     void receive();
 
+    static void maybeReceive(std::weak_ptr<Mailbox>);
+
 private:
     Scheduler& scheduler;