libsframe: fix checks in flip_fde

Adjust the sanity checks for flip_fde workflow and optional trailing
section padding to account for the case of ihp->sfh_fdeoff != 0 or
ihp->sfh_freoff != total FDEs size.

Reviewed-by: Jens Remus <jremus@linux.ibm.com>

libsframe/
        * sframe.c (flip_sframe): Fix checks in flip_fde to accommodate
	cases when sfh_fdeoff != 0 or when SFrame FREs are placed after
	a gap from SFrame FDEs.