Reject dependency and option_dependency containing null characters in C++ DescriptorPool

`DescriptorBuilder::BuildFileImpl` validates that `FileDescriptorProto.name`,
`package`, and symbol names do not contain embedded null characters (`'\0'`),
but previously did not check `dependency` or `option_dependency`. As a result,
`BuildFile` accepted `option_dependency` (and unknown/weak/lazy `dependency`)
strings with embedded null characters, creating placeholder `FileDescriptor`s
with null characters in their names or truncating/failing during lazy
dependency initialization in `InternalDependenciesOnceInit`.

PiperOrigin-RevId: 1000977826

src/google/protobuf/descriptor.cc

diff --git a/src/google/protobuf/descriptor.cc b/src/google/protobuf/descriptor.cc
index 742ac12..e0fe270 100644
--- a/src/google/protobuf/descriptor.cc
+++ b/src/google/protobuf/descriptor.cc
@@ -5571,7 +5571,8 @@
             i >= proto.dependency_size()
                 ? proto.option_dependency(i - proto.dependency_size())
                 : proto.dependency(i);
-        if (tables_->FindFile(name) == nullptr &&
+        if (!absl::StrContains(name, '\0') &&
+            tables_->FindFile(name) == nullptr &&
             (pool_->underlay_ == nullptr ||
              pool_->underlay_->FindFileByName(name) == nullptr)) {
           // We don't care what this returns since we'll find out below anyway.
@@ -5725,6 +5726,12 @@
     absl::string_view name =
         is_option_dep ? proto.option_dependency(i - proto.dependency_size())
                       : proto.dependency(i);
+    if (absl::StrContains(name, '\0')) {
+      AddError(name, proto, DescriptorPool::ErrorCollector::IMPORT, [&] {
+        return absl::StrCat("\"", name, "\" contains null character.");
+      });
+      return nullptr;
+    }
     if (!seen_dependencies.insert(name).second) {
       AddTwiceListedError(proto, name);
     }

src/google/protobuf/descriptor_unittest.cc

diff --git a/src/google/protobuf/descriptor_unittest.cc b/src/google/protobuf/descriptor_unittest.cc
index 9cd95ef..eb3bb73 100644
--- a/src/google/protobuf/descriptor_unittest.cc
+++ b/src/google/protobuf/descriptor_unittest.cc
@@ -5395,6 +5395,25 @@
                  "character.\n"));
 }
 
+TEST_F(ValidationErrorTest, NullCharDependency) {
+  pool_.AllowUnknownDependencies();
+  BuildFileWithErrors(
+      "name: \"bar.proto\" "
+      "dependency: \"foo\\000\\001\\013.proto\"",
+      STATIC_STR("bar.proto: foo\0\x1\v.proto: IMPORT: "
+                 "\"foo\0\x1\v.proto\" contains null character.\n"));
+}
+
+TEST_F(ValidationErrorTest, NullCharOptionDependency) {
+  BuildFileWithErrors(
+      "name: \"bar.proto\" "
+      "syntax: \"editions\" "
+      "edition: EDITION_2024 "
+      "option_dependency: \"foo\\000\\001\\013.proto\"",
+      STATIC_STR("bar.proto: foo\0\x1\v.proto: IMPORT: "
+                 "\"foo\0\x1\v.proto\" contains null character.\n"));
+}
+
 TEST_F(ValidationErrorTest, MissingFileName) {
   BuildFileWithErrors("",