HBSD: Fix mismerge

Typo.
Signed-off-by: Shawn Webb <shawn.webb@hardenedbsd.org>

lib/libthr/thread/thr_stack.c

@@ -262,7 +262,7 @@ _thr_stack_alloc(struct pthread_attr *attr)
 		 * Allocate a stack from or below usrstack, depending
 		 * on the LIBPTHREAD_BIGSTACK_MAIN env variable.
 		 */
-		if (last_stack == NULL)
+		if (last_stack == NULL) {
 			last_stack = _usrstack - _thr_stack_initial -
 			    _thr_guard_default;
 		}