libsframe: s390: No further decode if sframe_get_fre_offset returns err

libsframe: s390: No further decode if sframe_get_fre_offset returns err

SFrame FREs without any offsets will later be used to represent
.cfi_undefined RA in SFrame.

As a result the API to get the CFA offset can return an error value, if
there are no offsets.  Do not apply the s390x-specific decoding of CFA
offset on the error return value.

libsframe/
	* sframe.c (sframe_fre_get_cfa_offset): Do not apply s390x-
	specific decoding to error return value.

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