Added wxMemoryBuffer::Clear() and IsEmpty() methods.

Add two convenient methods which may be used instead of testing or setting the
buffer length.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70407 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/buffer.h

@@ -535,6 +535,8 @@ public:
     size_t GetBufSize() const { return m_bufdata->m_size; }
     size_t GetDataLen() const { return m_bufdata->m_len; }
 
+    bool IsEmpty() const { return GetDataLen() == 0; }
+
     void   SetBufSize(size_t size) { m_bufdata->ResizeIfNeeded(size); }
     void   SetDataLen(size_t len)
     {
@@ -542,6 +544,8 @@ public:
         m_bufdata->m_len = len;
     }
 
+    void Clear() { SetDataLen(0); }
+
     // Ensure the buffer is big enough and return a pointer to it
     void *GetWriteBuf(size_t sizeNeeded)
     {