6759775: RegularFileObject.inferBinaryName gives bad result on empty path

Reviewed-by: mcimadamore

src/share/classes/com/sun/tools/javac/file/RegularFileObject.java

@@ -89,6 +89,8 @@ class RegularFileObject extends BaseFileObject {
         for (File dir: path) {
             //System.err.println("dir: " + dir);
             String dPath = dir.getPath();
+            if (dPath.length() == 0)
+                dPath = System.getProperty("user.dir");
             if (!dPath.endsWith(File.separator))
                 dPath += File.separator;
             if (fPath.regionMatches(true, 0, dPath, 0, dPath.length())