Fix an assert.

Fix an assert.
This occured when we were chasing a branch insn (thereby setting the
disassembly result to Dis_ResteerU and the continueAt field to something
non-zero) and later changing the result kind to Dis_StopHere (because
the next insn is an EX insn). The ContinueAt field remained non-zero
in the case causing an assert down the road.
This should fix the failing test memcheck/tests/linux/timerfd-syscall

git-svn-id: svn://svn.valgrind.org/vex/trunk@2186