[wasm] Fix start merge arity of function block

This value was uninitialized before. Initialize it to zero. Also, fix
the tracing output to actually print the start arity and not the end
arity.

R=rossberg@chromium.org

Change-Id: I1eda3be88ca842f60e40e3fb630eca254619ae83
Reviewed-on: https://chromium-review.googlesource.com/756702Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49176}

src/wasm/function-body-decoder-impl.h

@@ -1283,6 +1283,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
     // Set up initial function block.
     {
       auto* c = PushBlock();
+      InitMerge(&c->start_merge, 0, [](uint32_t) -> Value { UNREACHABLE(); });
       InitMerge(&c->end_merge,
                 static_cast<uint32_t>(this->sig_->return_count()),
                 [&] (uint32_t i) {
@@ -1799,7 +1800,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
             default:
               break;
           }
-          if (c.start_merge.arity) PrintF("%u-", c.end_merge.arity);
+          if (c.start_merge.arity) PrintF("%u-", c.start_merge.arity);
           PrintF("%u", c.end_merge.arity);
           if (!c.reachable()) PrintF("%c", c.unreachable() ? '*' : '#');
         }