dm: fix dm_substitute_devices()

When candidate is the last parameter, candidate_end points to the '\0'
character and not the DM_FIELD_SEP character.  In such a situation, we
should not move the candidate_end pointer one character backward.

(Cherry-picked from https://android-review.googlesource.com/#/c/221680/

)
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
Change-Id: Ifc042e07da99fcfdfd46811d4208b39b427a6d99