Fixes crashing bug in FixedQueue. (#859)

* Fixes crashing bug in FixedQueue.
When there were many noncommit_back() writes, the full() function
would still return that there is open space at the end of the queue.
This would then cause certian append function to overrun the end
of the space.

* fix whitespace

Author: balazsracz

src/freertos_drivers/common/FixedQueue.hxx

@@ -64,7 +64,20 @@ public:
     /// @return true if there is no entry in the queue.
     bool empty() { return size() == 0; }
     /// @return true if the queue cannot accept more elements.
-    bool full() { return size() >= SIZE; }
+    bool full()
+    {
+        auto sz = size();
+        if (sz >= SIZE)
+        {
+            return true;
+        }
+        if (sz != 0 && rdIndex_ == wrIndex_)
+        {
+            // noncommit members make the queue full.
+            return true;
+        }
+        return false;
+    }
     /// @return the current number of entries in the queue.
     size_t size() { return __atomic_load_n(&count_, __ATOMIC_SEQ_CST); }