armv7a: fix scan-build warnings

armv7a: fix scan-build warnings

"Value stored to 'retval' is never read": Check and propagate error
"Dereference of null pointer": Probably bogus, maybe triggered by the null
check on armv7a, so remove the check since it can't be null anyway.

Change-Id: I3bc44e52af1589ff40e6a42deda0ce7f3a25e397
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/119
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>