RFR 8166358: Re-enable String verification in java_lang_String::create_from_str()

[coleen.phillimore at oracle.com]

Summary: Check for invalid strings in class names in debug mode, and 
only verify valid strings in create_for_str().

See bug for more information on why this commented out code needs to be 
conditional but not commented out.  Tested with mach5 tier1-6, and new 
test for new -Xcheck:jni check.

open webrev at http://cr.openjdk.java.net/~coleenp/2020/8166358.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8166358

Thanks,
Coleen