Fix self.decode typo

[bysiber]

In ImageFile.Parser, the class uses self.decoder throughout (initialized on line 516, checked on line 545, set on line 590). But line 584 assigns self.decode = None (missing the trailing "r"), which creates a new attribute that's never referenced anywhere.

The intent is clearly to reset self.decoder to None when the image has custom load code or multiple tiles, so the parser falls through to the elif self.image: branch on subsequent feed() calls.

[radarhere]

I've created bysiber#1 with a suggestion to simplify the code.

[hugovk]

Rather than:

if condition:
    pass
else:
    do_thing()

Invert and drop the else:

if not condition:
    do_thing()

(And rewrite the condition.)

[bysiber]

Good call, inverted the condition so it reads more naturally. Pushed.