s390/mem_detect: do not update output parameters on failure

Function __get_mem_detect_block() resets start and end
output parameters in case of invalid mem_detect array
index is provided. That violates the rule of sparing
the output on fail path and leads e.g to a below anomaly:

	for_each_mem_detect_block(i, &start, &end)
		continue;

One would expect start and end contain addresses of the
last memory block (if available), but in fact the two
will be reset to zeroes. That is not how an iterator is
expected to work.

Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>