parser: Fix XML_ERR_UNSUPPORTED_ENCODING errors

Commit 45157261 added the check in the wrong place.

Also allow unsupported encoding in xmlNewInputInternal.

Fixes #654.

error.c

@@ -1161,6 +1161,9 @@ xmlErrString(xmlParserErrors code) {
         case XML_ERR_REDECL_PREDEF_ENTITY:
             errmsg = "Invalid redeclaration of predefined entity";
             break;
+        case XML_ERR_UNSUPPORTED_ENCODING:
+            errmsg = "Unsupported encoding";
+            break;
 
         case XML_IO_UNKNOWN:
             errmsg = "Unknown IO error"; break;