swupdate-ab: improve error handling if mnt_point is empty

swupdate-ab: improve error handling if mnt_point is empty

Depending on where the error occurs, mnt_point can be empty. If
this is the case, this error can happen:

Status: 4 message: ERROR util.c : read_lines_notify : 1166 : swupdate-ab.sh: Error: destination partition "/dev/mmcblk0p2" not found
Status: 4 message: ERROR util.c : read_lines_notify : 1166 : swupdate-ab.sh: Error 1 occurred
Status: 4 message: ERROR util.c : read_lines_notify : 1166 : BusyBox v1.36.1 () multi-call binary.
Status: 4 message: ERROR util.c : read_lines_notify : 1166 : Usage: grep [-HhnlLoqvsrRiwFE] [-m N] [-A|B|C N] { PATTERN | -e PATTERN... | -f FILE... } [FILE]...
Status: 2 message: [__run_cmd] : /tmp/scripts/swupdate-ab.sh preinst p2 command returned 1

Add check for valid mnt_point before trying to unmount.