Enable -Werror=conditional-uninitialized if available

And fix two trivial local violations - initialized to the empty or
failing value, respectively.

Note, in the current code these are false positives as the called
functions do set the value in all cases, but these initializations
remove the reliance on that external property.

CMakeLists.txt

@@ -264,6 +264,13 @@ if(BUILD_SHARED_LIBS)
   )
 endif(BUILD_SHARED_LIBS)
 
+check_cxx_compiler_flag(-Werror=conditional-uninitialized HAVE_CONDITIONAL_UNINITIALIZED)
+if(HAVE_CONDITIONAL_UNINITIALIZED)
+  target_compile_options(leveldb
+    PUBLIC
+      -Werror=conditional-uninitialized)
+endif(HAVE_CONDITIONAL_UNINITIALIZED)
+
 if(HAVE_CLANG_THREAD_SAFETY)
   target_compile_options(leveldb
     PUBLIC

db/corruption_test.cc

@@ -157,7 +157,7 @@ class CorruptionTest : public testing::Test {
 
   int Property(const std::string& name) {
     std::string property;
-    int result;
+    int result = -1;
     if (db_->GetProperty(name, &property) &&
         sscanf(property.c_str(), "%d", &result) == 1) {
       return result;

db/version_set.cc

@@ -994,7 +994,7 @@ bool VersionSet::ReuseManifest(const std::string& dscname,
   }
   FileType manifest_type;
   uint64_t manifest_number;
-  uint64_t manifest_size;
+  uint64_t manifest_size = 0;
   if (!ParseFileName(dscbase, &manifest_number, &manifest_type) ||
       manifest_type != kDescriptorFile ||
       !env_->GetFileSize(dscname, &manifest_size).ok() ||