[wasm] Fix "unreachable" code for invalid modules

Better return a validation error than assume that invalid modules
can't reach invalid code paths. ClusterFuzz prefers errors over
crashes.

Bug: chromium:1084389
Change-Id: I67f0c2f48022cff8a7821901ddfdd42fa2c4a5c6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2209059
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67920}

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

@@ -1636,7 +1636,9 @@ class WasmDecoder : public Decoder {
             return 2;
 
           default:
-            UNREACHABLE();
+            // This is unreachable except for malformed modules.
+            decoder->error(pc, "invalid gc opcode");
+            return 2;
         }
       }
       default: