RFR(xs): 8155574: ClassLoader::initialize_module_loader_map crashes if the char_buf is not NULL terminated

[Calvin Cheung]

Please review this small change for fixing char_buf may not be NULL 
terminated.
Instead of finding the length of char_buf by using strlen(), this fix 
uses the size returned from JImageFindResource().

bug: https://bugs.openjdk.java.net/browse/JDK-8155574
webrev: http://cr.openjdk.java.net/~ccheung/8155574/webrev.00/

Testing:
     JPRT
     RBT on hotspot_runtime

thanks,
Calvin