Bug 1319009 - View Source shows a blank New Tab with a forced charset. r=mconley,mossop

MozReview-Commit-ID: 7P7gdH8Ltmm

--HG--
extra : rebase_source : de7ba0eaa280d6e539489d01c62ef5831fa88282

toolkit/components/viewsource/content/viewSource-content.js

@@ -291,7 +291,9 @@ var ViewSourceContent = {
     const viewSrcURL = "view-source:" + URL;
 
     if (forcedCharSet) {
-      docShell.charset = forcedCharSet;
+      try {
+        docShell.charset = forcedCharSet;
+      } catch (e) { /* invalid charset */ }
     }
 
     if (lineNumber && lineNumber > 0) {