RFR(XS) 8190809 JVM crashes while generating appcds for classpath with empty directory entry

[Ioi Lam]

Hi

Please review this very small change. I hope to invoke the trivial rule 
so that I can push before today's 8PM PDT deadline for JDK 10 integration.

https://bugs.openjdk.java.net/browse/JDK-8190809
http://cr.openjdk.java.net/~iklam/jdk10/8190809-cds-crash-empty-cp.v01/

The crash happened because CDS was trying to use a classpath as a JAR 
file, before checking it's actually a JAR file.

I added test case for the use of directories in -cp during CDS dumping, 
both for empty and non-empty cases.

I will test with tier1/tier2 testing before pushing.

Thanks
- Ioi






PS: the check for non-emptiness is done in here:

void ClassLoader::check_shared_classpath(const char *path) {
   if (strcmp(path, "") == 0) {
     exit_with_path_failure("Cannot have empty path in archived 
classpaths", NULL);
   }

   struct stat st;
   if (os::stat(path, &st) == 0) {
     if ((st.st_mode & S_IFMT) != S_IFREG) { // is not a regular file
       if (!os::dir_is_empty(path)) {
         tty->print_cr("Error: non-empty directory '%s'", path);
         exit_with_path_failure("CDS allows only empty directories in 
archived classpaths", NULL);
       }
     }
   }
}