extmod/modframebuf: Fix signed/unsigned comparison pendantic warning.
extmod/modframebuf: Fix signed/unsigned comparison pendantic warning.

Happened with 32-bit gcc 4.8.4.

adafruit/micropython,adafruit/circuitpython,henriknelson/micropython,TDAbboud/micropython,kerneltask/micropython,hiway/micropython,deshipu/micropython,MrSurly/micropython,chrisdearman/micropython,adafruit/micropython,MrSurly/micropython,HenrikSolver/micropython,MrSurly/micropython-esp32,pfalcon/micropython,pramasoul/micropython,selste/micropython,oopy/micropython,Timmenem/micropython,alex-robbins/micropython,selste/micropython,torwag/micropython,alex-robbins/micropython,pfalcon/micropython,AriZuu/micropython,lowRISC/micropython,bvernoux/micropython,Timmenem/micropython,chrisdearman/micropython,MrSurly/micropython-esp32,SHA2017-badge/micropython-esp32,hiway/micropython,dmazzella/micropython,MrSurly/micropython,henriknelson/micropython,pramasoul/micropython,blazewicz/micropython,dmazzella/micropython,MrSurly/micropython,oopy/micropython,HenrikSolver/micropython,ryannathans/micropython,micropython/micropython-esp32,tobbad/micropython,trezor/micropython,pfalcon/micropython,swegener/micropython,PappaPeppar/micropython,tralamazza/micropython,ryannathans/micropython,chrisdearman/micropython,trezor/micropython,infinnovation/micropython,pozetroninc/micropython,PappaPeppar/micropython,micropython/micropython-esp32,Timmenem/micropython,alex-robbins/micropython,trezor/micropython,lowRISC/micropython,adafruit/circuitpython,tralamazza/micropython,micropython/micropython-esp32,TDAbboud/micropython,pozetroninc/micropython,SHA2017-badge/micropython-esp32,pramasoul/micropython,bvernoux/micropython,AriZuu/micropython,swegener/micropython,lowRISC/micropython,MrSurly/micropython,alex-robbins/micropython,infinnovation/micropython,swegener/micropython,pfalcon/micropython,infinnovation/micropython,pfalcon/micropython,bvernoux/micropython,MrSurly/micropython-esp32,ryannathans/micropython,bvernoux/micropython,micropython/micropython-esp32,henriknelson/micropython,tralamazza/micropython,tobbad/micropython,henriknelson/micropython,torwag/micropython,TDAbboud/micropython,tobbad/micropython,adafruit/circuitpython,adafruit/circuitpython,kerneltask/micropython,swegener/micropython,adafruit/circuitpython,PappaPeppar/micropython,trezor/micropython,selste/micropython,pramasoul/micropython,AriZuu/micropython,dmazzella/micropython,MrSurly/micropython-esp32,tobbad/micropython,torwag/micropython,AriZuu/micropython,Timmenem/micropython,blazewicz/micropython,chrisdearman/micropython,blazewicz/micropython,oopy/micropython,torwag/micropython,dmazzella/micropython,HenrikSolver/micropython,TDAbboud/micropython,hiway/micropython,Timmenem/micropython,henriknelson/micropython,tralamazza/micropython,pramasoul/micropython,ryannathans/micropython,TDAbboud/micropython,SHA2017-badge/micropython-esp32,ryannathans/micropython,adafruit/circuitpython,HenrikSolver/micropython,adafruit/micropython,pozetroninc/micropython,SHA2017-badge/micropython-esp32,blazewicz/micropython,deshipu/micropython,oopy/micropython,hiway/micropython,infinnovation/micropython,infinnovation/micropython,deshipu/micropython,alex-robbins/micropython,swegener/micropython,adafruit/micropython,lowRISC/micropython,torwag/micropython,trezor/micropython,blazewicz/micropython,deshipu/micropython,MrSurly/micropython-esp32,kerneltask/micropython,PappaPeppar/micropython,selste/micropython,bvernoux/micropython,hiway/micropython,lowRISC/micropython,PappaPeppar/micropython,adafruit/micropython,AriZuu/micropython,HenrikSolver/micropython,selste/micropython,kerneltask/micropython,tobbad/micropython,pozetroninc/micropython,oopy/micropython,micropython/micropython-esp32,pozetroninc/micropython,SHA2017-badge/micropython-esp32,kerneltask/micropython,deshipu/micropython,chrisdearman/micropython