8190497: DHParameterSpec.getL() returns zero after JDK-8072452

Reviewed-by: xuelei

src/share/classes/com/sun/crypto/provider/DHParameterGenerator.java

@@ -133,6 +133,9 @@ public final class DHParameterGenerator extends AlgorithmParameterGeneratorSpi {
     @Override
     protected AlgorithmParameters engineGenerateParameters() {
 
+        if (this.exponentSize == 0) {
+            this.exponentSize = this.primeSize - 1;
+        }
         if (random == null) {
             random = SunJCE.getRandom();
         }