mips: fix for mips-disassembler when branch is at block_size-2 position

Check if the last instruction in the block is a branch or jump instruction
should happen only if the disassembler was not already stopped.
Incorrect conditional led to a boundary case in which jumps/branches were not
executed when placed on "max_insns - 2" position in the block.
none/tests/mips32/block_size test will be added to Valgrind to describe the case
and check for regressions in future.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2648 8f6e269a-dfd6-0310-a8e1-e2731360e62c