rtos: fix uninitialised variable warning

Breaks build with clang 3.3. This "addr" variable is actually always
ignored by next_symbol() when cur_symbol is an empty string but clang
can't (and probably shouldn't) prove that automatically.

Change-Id: Id030f1aa34b9d40b5fa20a422031511520e52669
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1876


Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>