libsframe: use const char * consistently for immutable FRE buffers

libsframe/
        * sframe.c (sframe_decode_fre): Use const char * datatype when
	handling buffer containing the FREs.
	(sframe_fre_get_end_ip_offset): Likewise.
	(sframe_find_fre): Likewise.
	(sframe_decoder_get_fre): Likewise.