Check if file (still)is present in the ZipComparator

If nested jars are compared the file of the baseline is only transient
and can't be accessed after the compare.

In this case, simply do not copy the files and instead assume it was
already written out as part of the outer comparison delta operation.

(cherry picked from commit e6dab28)

tycho-artifactcomparator/src/main/java/org/eclipse/tycho/zipcomparator/internal/ZipComparatorImpl.java

@@ -17,6 +17,7 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.nio.file.Files;
+import java.nio.file.StandardCopyOption;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.HashSet;
@@ -99,9 +100,16 @@ public ZipArtifactDelta(Map<String, ? extends ArtifactDelta> members, File basel
 
         @Override
         public void writeDetails(File basedir) throws IOException {
+            basedir.mkdirs();
             super.writeDetails(basedir);
-            Files.copy(baseline.toPath(), basedir.toPath().resolve("baseline-" + baseline.getName()));
-            Files.copy(reactor.toPath(), basedir.toPath().resolve("build-" + reactor.getName()));
+            if (baseline.isFile()) {
+                Files.copy(baseline.toPath(), basedir.toPath().resolve("baseline-" + baseline.getName()),
+                        StandardCopyOption.REPLACE_EXISTING);
+            }
+            if (reactor.isFile()) {
+                Files.copy(reactor.toPath(), basedir.toPath().resolve("build-" + reactor.getName()),
+                        StandardCopyOption.REPLACE_EXISTING);
+            }
         }
 
     }