Handle some illegal characters in hex string

Do not throw exception when hex strings are in the wrong format

Currently pdf.js is throwing an exception for the following hex string:

`<7 0 2 15 5 2 2 2 4 3 2 4>`

The issue is that the 15 is not a valid hex character so pdf.js ends up
throwing an exception.

This diff changes the parser to process the above hex string as follow:

`70 21 55 2 24 32` (Note: the final 4 of the hex string is ignored)

replicating the behaviour of MuPDF, and doesn't throw an exception.
Merge pull request #2540 from mduan/handle-broken-hex-string

Handle broken hex string