Fix several warnings

A small, nonexhaustive set of warning fixes. The DiscIO Volume change
is a workaround for a GCC bug [1] that causes returning an unengaged
std::optional to emit annoying -Wmaybe-uninitialized warnings.
This last change alone fixes pages upon pages of warnings since
Volume.h is included from several files.

-Wstringop-truncation is another irrelevant warning for us, but
unfortunately there seems to be no way to disable it without
adding ugly pragmas wherever the warning appears.

Source/Core/DiscIO/Blob.h

@@ -53,7 +53,7 @@ public:
   {
     T temp;
     if (!Read(offset, sizeof(T), reinterpret_cast<u8*>(&temp)))
-      return {};
+      return std::nullopt;
     return Common::FromBigEndian(temp);
   }
 

Source/Core/DiscIO/DirectoryBlob.cpp

@@ -451,7 +451,7 @@ void DirectoryBlobReader::SetWiiRegionData(const std::string& game_partition_roo
     ERROR_LOG(DISCIO, "Couldn't read age ratings from %s", region_bin_path.c_str());
 
   constexpr u64 WII_REGION_DATA_ADDRESS = 0x4E000;
-  constexpr u64 WII_REGION_DATA_SIZE = 0x20;
+  [[maybe_unused]] constexpr u64 WII_REGION_DATA_SIZE = 0x20;
   m_nonpartition_contents.Add(WII_REGION_DATA_ADDRESS, m_wii_region_data);
 }
 

Source/Core/DiscIO/Volume.h

@@ -50,7 +50,7 @@ public:
   {
     T temp;
     if (!Read(offset, sizeof(T), reinterpret_cast<u8*>(&temp), partition))
-      return {};
+      return std::nullopt;
     return Common::FromBigEndian(temp);
   }
   std::optional<u64> ReadSwappedAndShifted(u64 offset, const Partition& partition) const

Source/Core/DiscIO/VolumeVerifier.cpp

@@ -88,11 +88,9 @@ void RedumpVerifier::Start(const Volume& volume)
       return {};
     }
 
-    DownloadStatus status;
     {
       std::lock_guard lk(download_state->mutex);
       download_state->status = DownloadDatfile(system, download_state->status);
-      status = download_state->status;
     }
 
     switch (download_state->status)