Fix delta_mm.e with zero extruders

[p3p]

Description

With 0 extruders the uninitialised delta_mm.e was used in some calculations which in rare cases could lead to a max_rate of 0. With ADAPTIVE_STEP_SMOOTHING enabled the silent error becomes obvious by freezing the board.

Related Issues

A comment by @MitchelBerberich on #15865

[sjasonsmith]

I reproduced the original failure and verified this fix on both an SKR 1.3 (LPC1768) and SKR Mini E3 DIP (STM32F1).

[sjasonsmith]

When reproducing this on my SKR 1.3, I couldn't get it to reboot when sending through Octoprint, I had to send directly through a serial terminal (per @p3p's suggestion).

I did notice that prior to this fix the behavior when sending through Octoprint was choppy, with frequent pauses in the movement. After this change it all seems smooth now. I can't tell if it was choppy when sending directly from my PC, because it would crash too soon to tell.

[MitchelBerberich]

Problem is solved for me, too: Compiled current version again without your fix => problem still there. Then applied your patch => everything runs smooth to the end of the gcode script. Thank you very much.

[anttix]

This seems to have fixed #15567 as well. Thank you!