RFR (XS) JDK-8041918: BootstrapMethods attribute cannot be empty

[Lois Foltan]

Hello,

Please review the following fix:

Webrev:
     http://cr.openjdk.java.net/~lfoltan/bug_jdk8041918/

Bug: BootstrapMethods attribute cannot be empty
     https://bugs.openjdk.java.net/browse/JDK-8041918

Summary of fix:
JVMS 8, section 4.7.23 details the BootstrapMethods attribute. In 
particular, page 155, describes the "attribute length" field of a 
BootstrapMethods attribute by documenting, "the value of the 
attribute_length item indicates the length of the attribute, excluding 
the initial six bytes."  Thus, a valid BootstrapMethod attribute could 
have an empty bootstrap_methods table where the num_bootstrap_methods 
field is equal to zero.  This situation would set the attribute_length 
legitimately to 2.  The class format check within 
ClassFileParser::parse_classfile_bootstrap_methods_attribte() must check 
for attribute_byte_length to be greater than or equal to the sizeof(u2). 
This check was added post JDK 8, an empty bootstrap_methods table was 
accepted by JDK 8 without generating a ClassFormatError.  Thank you to 
Harold Seigel for helping to figure out the .jcod files to yield a 
reproducer for this bug.

Tests:
     Hotspot's jtreg tests, JCK lang & vm, JDK lang & util, 
vm.quick.testlist

Thank you,
Lois