Add stricter validation in `Catalog.readPageLabels`
Add stricter validation in `Catalog.readPageLabels`

The current PageLabel dictionary validation code won't catch some (unlikely) forms of corruption. For example: a `Type`/`S` entry being `null`/`0`/empty string, a `P`/`St` entry being `null`/`0`.

Please note: I'm not aware of any bugs caused by the old code, but I've had this patch sitting locally for some time and figured it couldn't hurt to submit it.

caremerge/pdf.js,macroplant/pdf.js,nawawi/pdf.js,macroplant/pdf.js,mukulmishra18/pdf.js,xavier114fch/pdf.js,Snuffleupagus/pdf.js,caremerge/pdf.js,nawawi/pdf.js,timvandermeij/pdf.js,showpad/mozilla-pdf.js,mozilla/pdf.js,Shoobx/pdf.js,macroplant/pdf.js,mainegreen/pdf.js,showpad/mozilla-pdf.js,nawawi/pdf.js,mukulmishra18/pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,Snuffleupagus/pdf.js,Shoobx/pdf.js,mainegreen/pdf.js,xavier114fch/pdf.js,Shoobx/pdf.js,mozilla/pdf.js,mozilla/pdf.js