Correctly clear out output UTF-16 buffer

Each character is more than one byte, so adjust the clearing of the
buffer to reflect that.

common/rfb/util.cxx

@@ -530,7 +530,7 @@ namespace rfb {
 
     // Alloc
     buffer = new wchar_t[sz];
-    memset(buffer, 0, sz);
+    memset(buffer, 0, sz * sizeof(wchar_t));
 
     // And convert
     out = buffer;