Merge pull request swiftlang#496 from adierking/tailq

shims: fix TAILQ_CONCAT() edge cases

Author: ktopley-apple

src/shims/generic_sys_queue.h

@@ -94,7 +94,11 @@
 
 #define TAILQ_CONCAT(head1, head2, field) do { \
 if (!TAILQ_EMPTY(head2)) { \
-(head1)->tq_last = (head2)->tq_first; \
+if ((head1)->tq_last) { \
+(head1)->tq_last->field.te_next = (head2)->tq_first; \
+} else { \
+(head1)->tq_first = (head2)->tq_first; \
+} \
 (head2)->tq_first->field.te_prev = (head1)->tq_last; \
 (head1)->tq_last = (head2)->tq_last; \
 TAILQ_INIT((head2)); \