virBitmapToString: Properly handle empty bitmaps

virBitmapNewEmpty() can create a bitmap with 0 length. With such a
bitmap virBitmapToString will return NULL rather than an empty string.
Initialize the buffer to avoid that.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/util/virbitmap.c

@@ -345,6 +345,9 @@ virBitmapToString(virBitmapPtr bitmap)
 
     sz = bitmap->map_len;
 
+    /* initialize buffer to return empty string for 0 length bitmap */
+    virBufferAdd(&buf, "", -1);
+
     while (sz--) {
         virBufferAsprintf(&buf, "%0*lx",
                           VIR_BITMAP_BITS_PER_UNIT / 4,