Prevent trying to map characters to the specials unicode block in adjustMapping (issue 4650)

Attempting to fix #4650.

In the PDF file in the issue a lot of the fonts have bogus `ToUnicode` dictionaries where all chars point to `0xFFFD`, i.e. inside the specials Unicode block `[0xFFF0, 0xFFFF]`. (This should also explain why even Adobe Reader is unable to copy most of the text in the document) 

Given that we currently ignore the specials Unicode block in other places (e.g. in `mapSpecialUnicodeValues`, [fonts.js#L503-L505](https://github.com/mozilla/pdf.js/blob/master/src/core/fonts.js#L503-L505)), it seemed reasonable to do the same thing here.

@yurydelendik In https://github.com/mozilla/pdf.js/pull/4671#issuecomment-41354479 you said that it was my turn to fix a font bug, so I've tried! :-D
Does this PR seem reasonable? If so, should I attach a test file?