This avoids to crash on this example:
\f0\fibi \fcharset0
-------^
Or:
\f0 \fibi\fcharset0
--^
Take into account that I found lots of RTF documents with this format:
\f0\fbidi \froman\fcharset0
\f0\froman\fprq2 \fcharset0
Which are not unsupported by this handler.
This was causing to parse a single space as an empty control
word with length 0, which was the source of several crashes.
Example:
\f0\fbidi \fcharset0
--------^
font index is 0, font family is bidi but when parsing charset we were
assuming control word was '' instead of 'charset0'.
This only fixes the crashes, the parseFontTable function works quite
awful right now.