Add null-pointer check after the last svm_range_new call. This was
originally reported by Zhou Qingyang <zhou1615@umn.edu> based on a
static analyzer.

To avoid duplicating the unwinding code from svm_range_handle_overlap,
I merged the two functions into one.
Signed-off-by: NFelix Kuehling <Felix.Kuehling@amd.com>
Cc: Zhou Qingyang <zhou1615@umn.edu>
Reviewed-by: NPhilip Yang <Philip.Yang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>