[BUG] Selectoed process out of bounds in grofer proc

[anihm136]

Describe the bug
When the process list changes for any reason and is refreshed, if the number of processes in the refreshed list is less than the index of the currently selected process, the selection goes out of bounds and no process is selected

To Reproduce
Steps to reproduce the behavior:

1. grofer proc
2. Scroll down to the last process
3. Wait for a while (or kill a process if you're impatient)
4. No process is selected

Expected behavior
If the selection goes out of bounds, cursor should reset to the last element in the new list

Screenshots

Before termination of last process

After termination of last process

Desktop (please complete the following information):

• OS: Linux 5.12.7-arch1-1
• Go Version: 1.16.4 linux/amd64

Additional context
Seems generally harmless for now, does not cause any crashes. Using any of the navigation keys from this state resets the selection to a valid position. However, this caused some issues on #112 for which a temporary fix has been made. Updating the selection using calcPos after every process refresh seems to fix the problem, but has not been tested for adverse behaviors or side effects