target: fix assert in 'monitor profile' on constant PC

target: fix assert in 'monitor profile' on constant PC

When target is stopped in WFI/WFE or is in an infinite loop, the
sampled PC will always return the same value.
Command 'profile' requires that distance between min and max PC
should be at least 2, which is not the case for constant PC, and
incorrectly enforces the check through as assert().

Move the code that reads the optional parameters 'start' and 'end'
and check the gap 'end - start' before running the profile.
For self-computed min and max, increase max (or decrease min) to
match the required constraint.
Drop the assert().

Change-Id: I2be8df8568ce8c889923888c492e4f7ce354b16b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: https://sourceforge.net/p/openocd/tickets/370/
Reviewed-on: https://review.openocd.org/c/openocd/+/7400
Tested-by: jenkins