Boot loop after updating to official Android 6.0 Marshmallow on my Nexus 5
If your device is in a boot-loop you will not be able to pull anything. 
You might want to try and booting the phone into bootloader IF possible by pressing volume down and power to power on the device.
I would recommend downloading the factory image from Google, extracting it and trying:

fastboot flash bootloader bootloader-hammerhead-hhz12k.img


To flash the bootloader image in in the zip via the command line, as well as boot and radio just to be safe. DO NOT FLASH userdata.img or cache.img, or run any flash all batch or script as this WILL erase everything that is currently on your device.

OR:

You can try to follow the same instructions to boot to bootloader and:

fastboot flash recovery twrp-2.8.7.1-hammerhead.img


To install TeamWin Recovery, which you can download here.
This will give you a custom recovery you can then boot to and possibly be able to adb pull from there.

Hope this helps!!