[wasm] Make lifting more robust for tags

The implicit import mechanism in Wasm needs a rework sooner or later.
For now, let's make it more robust in this case.

Fixed: 498266575
Change-Id: I17cc0021cbe945d5db16029c451e3ea6bfb55ff1
Reviewed-on: https://chrome-internal-review.googlesource.com/c/v8/fuzzilli/+/9159837
Auto-Submit: Matthias Liedtke <mliedtke@google.com>
Commit-Queue: Michael Achenbach <machenbach@google.com>
Reviewed-by: Michael Achenbach <machenbach@google.com>

Author: Liedtke

Sources/Fuzzilli/Lifting/WasmLifter.swift

@@ -772,7 +772,7 @@ public class WasmLifter {
                 temp += mutability ? [0x1] : [0x0]
                 continue
             }
-            if type.Is(.object(ofGroup: "WasmTag")) {
+            if type.Is(.object(ofGroup: "WasmTag")) && signature != nil  {
                 temp += [0x4, 0x0] + Leb128.unsignedEncode(try getSignatureIndex(signature!))
                 continue
             }