Allows preview callbacks to modify multiple buffers in order to show the
preview. Previously, if multiple buffers were modified, only the current
buffer would have its state restored. After this change, all buffers
have their state restored after preview.
Closes#19103.
clang: Result of operation is garbage or undefined
clang: Uninitialized argument value
Also check for == 's' instead of != 'n' as it is more straightforward.
Also fix another unrelated PVS warning:
PVS/V1071: Return value of win_comp_pos() is not always used
