tool_cb_prg: Fix integer overflow in progress bar

Commit 61faa0b4

 fixed the progress bar
width calculation to avoid integer overflow, but failed to account for
the fact that initial_size is initialized to -1 when the file size is
retrieved from the remote on an upload, causing another signed integer
overflow.  Fix by separately checking for this case before the width
calculation.

Closes #3984
Reported-by: Brian Carpenter (Geeknik Labs)
Reviewed-by: Daniel Stenberg <daniel@haxx.se>