common: Add FrameAllocator default constructor.

Also bool empty() and reset().

src/common/frameallocator.h

@@ -6,6 +6,13 @@
 class FrameAllocator
 {
 public:
+   FrameAllocator() :
+      mBase(nullptr),
+      mSize(0),
+      mOffset(0)
+   {
+   }
+
    FrameAllocator(void *base, size_t size) :
       mBase(reinterpret_cast<uint8_t *>(base)),
       mSize(size),
@@ -13,6 +20,12 @@ public:
    {
    }
 
+   bool
+   empty() const
+   {
+      return mOffset == 0;
+   }
+
    uint8_t *
    top() const
    {
@@ -36,6 +49,12 @@ public:
       return result;
    }
 
+   void
+   reset()
+   {
+      mOffset = 0;
+   }
+
 protected:
    uint8_t *mBase;
    size_t mSize;