dalvik: Fix compile failures with -DNDEBUG and gcc >= 4.6

With -DNDEBUG and -DLOG_NDEBUG, assert() and friends are defined to
nothing - causing variables that are used only in the context of assert
or logging to be defined, but not used, resulting in a build failure
if -Werror is enabled.

Change-Id: Iae18e0274ec2f11f95f918b75006be7cf375a8c0
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>

Conflicts:
	vm/Thread.cpp