[wasm] Fix DCHECK

The DCHECK is supposed to check that the read in the next line does not
read out of bounds. Hence we have to use {pc} and not {pc_}.
I accidentally triggered this while writing an unrelated test case, but
now fail to reproduce.

R=ahaas@chromium.org

Change-Id: Ia3fa9a9874866dc14180b22b9af526459fbda495
Reviewed-on: https://chromium-review.googlesource.com/568487Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46621}

src/wasm/decoder.h

@@ -302,7 +302,7 @@ class Decoder {
     const bool at_end = checked && pc >= end_;
     byte b = 0;
     if (!at_end) {
-      DCHECK_LT(pc_, end_);
+      DCHECK_LT(pc, end_);
       b = *pc;
       TRACE_IF(trace, "%02x ", b);
       result = result | ((static_cast<IntType>(b) & 0x7f) << shift);