bug#16753: 24.3.50; format function doesn't pad properly when given a fontified string

[Oleh]

Reproducible with emacs -q.
Here's the code, each line is followed by its result of evaluation in *scratch*:

    (setq foo #("overlay-put org-speedbar-restriction-lock-overlay          
..." 12 49 (fontified t) 50 55 (fontified t)))
    #("overlay-put org-speedbar-restriction-lock-overlay            ..." 12 49 
(fontified t) 50 55 (fontified t))
    
    (length foo)
    58
    
    (insert (format "%-60s %s" foo "foo"))
    overlay-put org-speedbar-restriction-lock-overlay       ... foonil
    
    (insert (format "%-61s %s" foo "foo"))
    overlay-put org-speedbar-restriction-lock-overlay       ... foonil
    
    (insert (format "%-62s %s" foo "foo"))
    overlay-put org-speedbar-restriction-lock-overlay       ... foonil

    (insert (format "%-66s %s" foo "foo"))
    overlay-put org-speedbar-restriction-lock-overlay       ...  foonil

Expected behavior is that lines should get consecutively larger, but
they don't until "%-66s %s".

regards,
Oleh