Fix exception when indexing arguments

When calling "fill(255, 255, 255)" with no alpha argument,
running on recent versions of the Javascriptcore engine
(e.g. in the Epiphany browser with WebKit 2.2.1),
Processing tries to access arguments[3] which throws an
exception. Apparently earlier versions of Javascriptcore
allowed this.
Merge pull request #74 from ptomato/arguments-fix

Fix exception when indexing arguments