Both methods decoded a LEB128 encoded integer, but only consume_leb
incremented the pc pointer accordingly.
This CL implements consume_leb by using checked_read_leb.

It also refactors a few things:
1) It removes error_pt, which was only avaible in checked_read_leb.
2) It renames the error method to errorf, since it receives a format
   string. This also avoids a name clash.
3) It implements sign extension directly in checked_read_leb instead of
   doing this in the caller.

R=ahaas@chromium.org
BUG=v8:5822

Change-Id: I8058f57418493861e5df26d4949041f6766d5138
Reviewed-on: https://chromium-review.googlesource.com/467150
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44405}