Reduce the number of `isCmd` calls slightly in the `XRef` class
Reduce the number of `isCmd` calls slightly in the `XRef` class

This reduces the total number of function calls, when reading the XRef table respectively when fetching uncompressed XRef entries.
Note in particular the `XRef.readXRefTable` method, where there're *two* back-to-back `isCmd` checks rather than just one.

mozilla/pdf.js,caremerge/pdf.js,macroplant/pdf.js,mainegreen/pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js,mainegreen/pdf.js,Shoobx/pdf.js,xavier114fch/pdf.js,nawawi/pdf.js,timvandermeij/pdf.js,nawawi/pdf.js,macroplant/pdf.js,Shoobx/pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,showpad/mozilla-pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,Shoobx/pdf.js,xavier114fch/pdf.js,mozilla/pdf.js,caremerge/pdf.js,mozilla/pdf.js,macroplant/pdf.js,Snuffleupagus/pdf.js,nawawi/pdf.js