Font view leaks on hilite moves w arrow keys

`fontforgeexe/fontview.c` routine `SCPreparePopup()` leaks strings on each call.
This can be provoked as simply as scanning through glyph cells using arrow
keys.  Not a big leak, less than 20 bytes per call.

Fix saves pointers and `free()`s the intermediate results strings.  It also frees the generated message after final use.