Merge bitcoin/bitcoin#34700: script: Fix undefined behavior in Clone() -- std::transform writes past end of empty vector

44feab23a7 script: Fix undefined behavior in Clone() -- std::transform writes past end of empty vector (Weixie Cui)

Pull request description:

  # Motivation
  This patch fixes undefined behavior in Clone() in src/script/descriptor.cpp.
  When std::transform is used with providers.begin() or subdescs.begin() as the output iterator, the vectors have been reserve()d but have size 0. Writing through begin() in that case writes past the logical end of the vector, which is undefined behavior.

ACKs for top commit:
  maflcko:
    lgtm ACK 44feab23a7
  rkrux:
    ACK 44feab23a7 because it gets rid of the possible undefined behaviour.
  frankomosh:
    Code Review ACK 44feab23a7. Fix seems minimal and correct.

Tree-SHA512: 8af3b6d97c139b32bd47d4c452b6b16befdaa7028a7bc1b6de0ab1f0a8cb35eb068710316a2c07fa60856e17e25307931aa3125b4f41d0fe7726b435483a52db

src/script/descriptor.cpp

@@ -1310,7 +1310,7 @@ public:
     {
         std::vector<std::unique_ptr<PubkeyProvider>> providers;
         providers.reserve(m_pubkey_args.size());
-        std::transform(m_pubkey_args.begin(), m_pubkey_args.end(), providers.begin(), [](const std::unique_ptr<PubkeyProvider>& p) { return p->Clone(); });
+        std::transform(m_pubkey_args.begin(), m_pubkey_args.end(), std::back_inserter(providers), [](const std::unique_ptr<PubkeyProvider>& p) { return p->Clone(); });
         return std::make_unique<MultisigDescriptor>(m_threshold, std::move(providers), m_sorted);
     }
 };
@@ -1533,7 +1533,7 @@ public:
     {
         std::vector<std::unique_ptr<DescriptorImpl>> subdescs;
         subdescs.reserve(m_subdescriptor_args.size());
-        std::transform(m_subdescriptor_args.begin(), m_subdescriptor_args.end(), subdescs.begin(), [](const std::unique_ptr<DescriptorImpl>& d) { return d->Clone(); });
+        std::transform(m_subdescriptor_args.begin(), m_subdescriptor_args.end(), std::back_inserter(subdescs), [](const std::unique_ptr<DescriptorImpl>& d) { return d->Clone(); });
         return std::make_unique<TRDescriptor>(m_pubkey_args.at(0)->Clone(), std::move(subdescs), m_depths);
     }
 };