libsframe: fix warning about argument of sframe_fre_sanity_check_p

libsframe: fix warning about argument of sframe_fre_sanity_check_p

Recent commit (6ca8915c) added a new API sframe_fre_get_ra_undefined_p
(). It has a 'const sframe_frame_row_entry *fre' argument, causing a
warning in function ‘sframe_fre_get_ra_undefined_p’:

libsframe/sframe.c:794:50: warning: passing argument 1 of
‘sframe_fre_sanity_check_p’ discards ‘const’ qualifier from pointer
target type [-Wdiscarded-qualifiers]
  794 |   if (fre == NULL || !sframe_fre_sanity_check_p (fre))
      |                                                  ^~~
libsframe/sframe.c:293:52: note: expected ‘sframe_frame_row_entry *’ but
argument is of type ‘const sframe_frame_row_entry *’
  293 | sframe_fre_sanity_check_p (sframe_frame_row_entry *frep)
      |                            ~~~~~~~~~~~~~~~~~~~~~~~~^~~~

Fix it by using const qualifier.

libsframe/
	* sframe.c (sframe_fre_sanity_check_p): Use const.