Fix #1569: Look for first non-static field in enum definitions to determine enum underlying type.

ICSharpCode.Decompiler/SRMExtensions.cs

@@ -69,13 +69,18 @@ namespace ICSharpCode.Decompiler
 				return false;
 			if (!baseType.IsKnownType(reader, KnownTypeCode.Enum))
 				return false;
-			var field = reader.GetFieldDefinition(typeDefinition.GetFields().First());
+			foreach (var handle in typeDefinition.GetFields()) {
+				var field = reader.GetFieldDefinition(handle);
+				if ((field.Attributes & FieldAttributes.Static) != 0)
+					continue;
 				var blob = reader.GetBlobReader(field.Signature);
 				if (blob.ReadSignatureHeader().Kind != SignatureKind.Field)
 					return false;
 				underlyingType = (PrimitiveTypeCode)blob.ReadByte();
 				return true;
 			}
+			return false;
+		}
 
 		public static bool IsDelegate(this TypeDefinitionHandle handle, MetadataReader reader)
 		{