Fixed a NRE when matching an Open Office XML format

src/FileSignatures/FileSignatures.csproj

@@ -13,8 +13,9 @@
     <GenerateAssemblyConfigurationAttribute>false</GenerateAssemblyConfigurationAttribute>
     <GenerateAssemblyCompanyAttribute>false</GenerateAssemblyCompanyAttribute>
     <GenerateAssemblyProductAttribute>false</GenerateAssemblyProductAttribute>
-    <Version>3.0.0</Version>
+    <Version>3.0.1</Version>
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
+    <AssemblyVersion>3.0.1.0</AssemblyVersion>
   </PropertyGroup>
 
   <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard1.6' Or '$(TargetFramework)' == 'netstandard2.0' ">

src/FileSignatures/Formats/OfficeOpenXml.cs

@@ -58,7 +58,10 @@ public override bool IsMatch(byte[] header)
                 }
                 finally
                 {
-                    archive.Dispose();
+                    if(archive != null)
+                    {
+                        archive.Dispose();
+                    }
                 }
             }
         }

test/FileSignatures.Tests/Formats/OfficeOpenXmlTests.cs

@@ -0,0 +1,23 @@
+using FileSignatures.Formats;
+using System.IO;
+using Xunit;
+
+namespace FileSignatures.Tests.Formats
+{
+    public class OfficeOpenXmlTests
+    {
+        [Fact]
+        public void InvalidZipArchive_DoesNotThrow()
+        {
+            var inspector = new FileFormatInspector();
+
+            using (var stream = new MemoryStream(new byte[] { 0x50, 0x4B, 0x03, 0x04 }))
+            {
+                var format = inspector.DetermineFileFormat(stream);
+
+                Assert.NotNull(format);
+                Assert.IsType<Zip>(format);
+            }
+        }
+    }
+}