fuzz: limit the number of nested wrappers in descriptors

The script building logic performs a quadratic number of copies in the
number of nested wrappers in the miniscript. Limit the number of nested
wrappers to avoid fuzz timeouts.

Thanks to Marco Falke for reporting the fuzz timeouts and providing a
minimal input to reproduce.

src/test/fuzz/descriptor_parse.cpp

@@ -76,6 +76,10 @@ FUZZ_TARGET(mocked_descriptor_parse, .init = initialize_mocked_descriptor_parse)
     // may perform quadratic operations on them. Limit the number of sub-fragments per fragment.
     if (HasTooManySubFrag(buffer)) return;
 
+    // The script building logic performs quadratic copies in the number of nested wrappers. Limit
+    // the number of nested wrappers per fragment.
+    if (HasTooManyWrappers(buffer)) return;
+
     const std::string mocked_descriptor{buffer.begin(), buffer.end()};
     if (const auto descriptor = MOCKED_DESC_CONVERTER.GetDescriptor(mocked_descriptor)) {
         FlatSigningProvider signing_provider;
@@ -90,6 +94,7 @@ FUZZ_TARGET(descriptor_parse, .init = initialize_descriptor_parse)
     // See comments above for rationales.
     if (HasDeepDerivPath(buffer)) return;
     if (HasTooManySubFrag(buffer)) return;
+    if (HasTooManyWrappers(buffer)) return;
 
     const std::string descriptor(buffer.begin(), buffer.end());
     FlatSigningProvider signing_provider;