Guard against nil field tokens in the localloc cpblk prefix

HandleCpblkInitializer already rejects fields with a nil metadata token
before casting to FieldDefinitionHandle; the localloc prefix path did
not, so a crafted assembly could make GetFieldDefinition throw instead
of the transform being skipped.

Assisted-by: Claude:claude-fable-5:Claude Code

ICSharpCode.Decompiler/IL/Transforms/TransformArrayInitializers.cs

@@ -518,6 +518,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					return;
 				if (!MatchGetStaticFieldAddress(src, out var field))
 					return;
+				if (field.MetadataToken.IsNil)
+					return;
 				var fd = context.PEFile.Metadata.GetFieldDefinition((FieldDefinitionHandle)field.MetadataToken);
 				if (!fd.HasFlag(System.Reflection.FieldAttributes.HasFieldRVA))
 					return;