Fix P/Invokes crashing in EverestModule.Load()

EverestModuleAssemblyContext.LoadUnmanagedFromThisMod(string) accesses
the module meta's Hash. If a P/Invoke occurs in EverestModule.Load(),
the Hash has not yet been computed, which crashes the game.

This commit fixes that by calculating the Hash if it is null.

Fixes EverestAPI#671.

Celeste.Mod.mm/Mod/Module/EverestModuleAssemblyContext.cs

@@ -520,7 +520,15 @@ private IntPtr LoadUnmanagedFromThisMod(string name) {
                     // We have to unzip the native libs into the cache
                     string cachePath = Path.Combine(Everest.Loader.PathCache, "unmanaged-libs", UnmanagedLibraryFolder, ModuleMeta.Name);
 
-                    string modHash = (ModuleMeta.Hash ?? ModuleMeta.Multimeta.First(meta => meta.Hash != null).Hash).ToHexadecimalString();
+                    string modHash = (
+                        ModuleMeta.Hash
+                        ?? ModuleMeta.Multimeta.FirstOrDefault(meta => meta.Hash != null).Hash
+
+                        // if we're here, this means a P/Invoke occured in Load(); calculate the hash real quick
+                        // https://github.com/EverestAPI/Everest/issues/761
+                        ?? (ModuleMeta.Hash = Everest.GetChecksum(ModuleMeta))
+                    ).ToHexadecimalString();
+
                     if (Directory.Exists(cachePath) && (!File.Exists(cachePath + ".sum") || File.ReadAllText(cachePath + ".sum") != modHash))
                         Directory.Delete(cachePath, true);
 

Celeste.Mod.mm/Mod/Module/EverestModuleMetadata.cs

@@ -126,7 +126,7 @@ public void PostParse() {
         /// </summary>
         internal void RegisterMod() {
             Everest.InvalidateInstallationHash();
-            Hash = Everest.GetChecksum(this);
+            Hash ??= Everest.GetChecksum(this);
 
             // Audio banks are cached, and as such use the module's hash. We can only ingest those now.
             if (patch_Audio.AudioInitialized) {