encoding: Don't shrink input too early in xmlCharEncOutput

Some exotic encodings like ISO646-FR don't support '#' characters, so
encoding a character reference can actually fail. Don't skip the
offending input in this case so the error will be reported on the next
call.

encoding.c

@@ -2491,7 +2491,6 @@ retry:
          */
         charrefLen = snprintf((char *) &charref[0], sizeof(charref),
                          "&#%d;", cur);
-        xmlBufShrink(in, len);
         xmlBufGrow(out, charrefLen * 4);
         c_out = xmlBufAvail(out);
         c_in = charrefLen;
@@ -2502,6 +2501,7 @@ retry:
             goto error;
         }
 
+        xmlBufShrink(in, len);
         xmlBufAddLen(out, c_out);
         writtentot += c_out;
         goto retry;