RFR (XXS): JDK-8138610 add assert to ThreadLocalAllocBuffer::clear_before_allocation to check the storage above top

[Joseph Provino]

Please review this one line change:

CR: JDK-8138610 <https://bugs.openjdk.java.net/browse/JDK-8138610> add 
assert to ThreadLocalAllocBuffer::clear_before_allocation to check the 
storage above top

Webrev: http://cr.openjdk.java.net/~jprovino/8138610/webrev.00

Tests:  Passes JPRT.

thanks.

joe