[test] Fix UBSan failure

UBSan starts complaining about a nullptr destination in memcpy after
https://crrev.com/c/2691828.
This CL fixes the error by not copying if there is nothing to copy.

R=nicohartmann@chromium.org

No-Try: true
No-Tree-Checks: true
Change-Id: I2c941b37d26931d6c2253bc3bb2c0aa659d4cb71
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2690605
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72690}

test/cctest/compiler/c-signature.h

@@ -100,8 +100,10 @@ class CSignatureOf : public CSignature {
     static_assert(
         std::is_same<decltype(*reps_), decltype(*param_types.data())>::value,
         "type mismatch, cannot memcpy");
-    memcpy(storage_ + kReturnCount, param_types.data(),
-           sizeof(*storage_) * kParamCount);
+    if (kParamCount > 0) {
+      memcpy(storage_ + kReturnCount, param_types.data(),
+             sizeof(*storage_) * kParamCount);
+    }
   }
 
  private: