libproc_impl.c previous_thr may be used uninitialized warning

When building with gcc7 on linux x86_64, and trying out different optimization options, I run into this error when
optimizing for space (-Os) :

/open_jdk/jdk_5/jdk/src/jdk.hotspot.agent/linux/native/libsaproc/libproc_impl.c: In function 'delete_thread_info':
/open_jdk/jdk_5/jdk/src/jdk.hotspot.agent/linux/native/libsaproc/libproc_impl.c:285:26: error: 'previous_thr' may be used uninitialized in this function [-Werror=maybe-uninitialized]
       previous_thr->next = current_thr->next;
       ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~

We should probably better initialize previous_thr to avoid this warning.