py/mpz: In mpz_as_str_inpl, convert always-false checks to assertions.
py/mpz: In mpz_as_str_inpl, convert always-false checks to assertions.

There are two checks that are always false so can be converted to (negated)
assertions to save code space and execution time.  They are:

1. The check of the str parameter, which is required to be non-NULL as per
   the original comment that it has enough space in it as calculated by
   mp_int_format_size.  And for all uses of this function str is indeed
   non-NULL.

2. The check of the base parameter, which is already required to be between
   2 and 16 (inclusive) via the assertion in mp_int_format_size.

henriknelson/micropython,pramasoul/micropython,bvernoux/micropython,infinnovation/micropython,henriknelson/micropython,selste/micropython,trezor/micropython,tralamazza/micropython,tobbad/micropython,pramasoul/micropython,pramasoul/micropython,ryannathans/micropython,MrSurly/micropython,tralamazza/micropython,swegener/micropython,adafruit/micropython,tralamazza/micropython,ryannathans/micropython,blazewicz/micropython,bvernoux/micropython,ryannathans/micropython,infinnovation/micropython,MrSurly/micropython,tobbad/micropython,pozetroninc/micropython,selste/micropython,infinnovation/micropython,swegener/micropython,bvernoux/micropython,adafruit/micropython,pfalcon/micropython,trezor/micropython,selste/micropython,selste/micropython,kerneltask/micropython,ryannathans/micropython,dmazzella/micropython,kerneltask/micropython,blazewicz/micropython,adafruit/circuitpython,MrSurly/micropython,adafruit/circuitpython,pramasoul/micropython,pozetroninc/micropython,swegener/micropython,blazewicz/micropython,MrSurly/micropython,kerneltask/micropython,MrSurly/micropython,adafruit/micropython,infinnovation/micropython,pfalcon/micropython,tobbad/micropython,adafruit/micropython,tralamazza/micropython,pfalcon/micropython,pozetroninc/micropython,ryannathans/micropython,selste/micropython,adafruit/micropython,pozetroninc/micropython,dmazzella/micropython,adafruit/circuitpython,tobbad/micropython,trezor/micropython,tobbad/micropython,trezor/micropython,kerneltask/micropython,swegener/micropython,dmazzella/micropython,henriknelson/micropython,blazewicz/micropython,pfalcon/micropython,bvernoux/micropython,bvernoux/micropython,swegener/micropython,henriknelson/micropython,dmazzella/micropython,pozetroninc/micropython,pramasoul/micropython,kerneltask/micropython,adafruit/circuitpython,blazewicz/micropython,adafruit/circuitpython,trezor/micropython,infinnovation/micropython,pfalcon/micropython,henriknelson/micropython,adafruit/circuitpython