fix: fail gracefully when running on a non-EPUB file (#1134)

Running EPUBCheck on a non-EPUB package file (for instance an Office
Open XML document) caused a a `NullPointerException`.
It now fails gracefully with a fatal error (RSC-002) potentially followed
by side-effect errors (typically RSC-001, PKG-006).

Fixes #1050

src/main/java/com/adobe/epubcheck/api/EpubCheck.java

@@ -268,10 +268,12 @@ void checkExtension(OCFPackage ocf, String extension)
         else
         {
           List<String> opfPaths = ocf.getOcfData().getEntries(OPFData.OPF_MIME_TYPE);
-          if(ocf.getOpfData().get(opfPaths.get(0)).getVersion() == EPUBVersion.VERSION_3) {
-            report.message(MessageId.PKG_024, EPUBLocation.create(epubFile.getName(), extension));
-          } else {
-            report.message(MessageId.PKG_017, EPUBLocation.create(epubFile.getName(), extension));
+          if (!opfPaths.isEmpty()) {
+            if(ocf.getOpfData().get(opfPaths.get(0)).getVersion() == EPUBVersion.VERSION_3) {
+              report.message(MessageId.PKG_024, EPUBLocation.create(epubFile.getName(), extension));
+            } else {
+              report.message(MessageId.PKG_017, EPUBLocation.create(epubFile.getName(), extension));
+            }
           }
         }
       }

src/main/java/com/adobe/epubcheck/xml/XMLParser.java

@@ -196,6 +196,9 @@ public void process()
       try
       {
         InputStream in = closer.register(context.resourceProvider.getInputStream(path));
+        if (in == null) {
+          return; // Abort processing. Missing required files are reported elsewhere.
+        }
         // System.err.println("DEBUG XMLParser#process on" + resource);
         if (!in.markSupported())
         {

src/test/java/com/adobe/epubcheck/api/Epub30CheckTest.java

@@ -367,4 +367,12 @@ public void testEdupubRenditions_Invalid_NoRenditionDCType()
     Collections.addAll(expectedErrors, MessageId.RSC_005);
     testValidateDocument("invalid/edupub-multiple-renditions-nodctype-rendition.epub");
   }
+
+  @Test
+  public void testNotAnEPUB()
+  {
+    Collections.addAll(expectedFatals, MessageId.RSC_002);
+    Collections.addAll(expectedErrors, MessageId.PKG_006, MessageId.RSC_001);
+    testValidateDocument("invalid/not-an-epub.docx");
+  }
 }