Bug: charFrequency is not defined in some cases

[loilo]

Trying to squash some cdnjs sources (e.g. angular-timer) with 0.4.6 introduced a new error.

It boils down to the freshly introduced program.charFrequency property not being initialized yet when the first calls to program.addWord() come in, throwing errors in that very method.

I'll attach a PR fixing this.

[nifgraup]

code merged, issue can be closed