[wasm] Fix serialization of WasmBeginCatch

The issue was introduced with
commit 7fb8254

While I was running the fuzzer for multiple hours, the fuzzer is more
persmissive in not crashing on invalid programs send over the wire, so
this wasn't detected.

Change-Id: I34f04902915539cb688c5c6eb6825d28a123ccb0
Reviewed-on: https://chrome-internal-review.googlesource.com/c/v8/fuzzilli/+/9130176
Reviewed-by: Michael Achenbach <machenbach@google.com>
Commit-Queue: Michael Achenbach <machenbach@google.com>
Auto-Submit: Matthias Liedtke <mliedtke@google.com>
Reviewed-by: Olivier Flückiger <olivf@google.com>

Author: Liedtke

Sources/Fuzzilli/FuzzIL/Instruction.swift

@@ -2532,7 +2532,7 @@ extension Instruction: ProtobufConvertible {
         }
 
         guard op.numInputs + op.numOutputs + op.numInnerOutputs == inouts.count else {
-            throw FuzzilliError.instructionDecodingError("incorrect number of in- and outputs")
+            throw FuzzilliError.instructionDecodingError("incorrect number of in- and outputs for operation \(op)")
         }
 
         opCache?.add(op)

Sources/Fuzzilli/FuzzIL/WasmOperations.swift

@@ -1421,7 +1421,7 @@ final class WasmBeginCatch : WasmOperation {
             requiredContext: [.wasmFunction])
     }
 
-    var blockOutputCount: Int {numInputs - 2}
+    var blockOutputCount: Int {numInputs - 3}
     var labelParameterCount: Int {numInnerOutputs - 2}
 }