Core: fixed a race resulting in extra sem_post()'s.

The mtx->wait counter was not decremented if we were able to obtain the lock
right after incrementing it.  This resulted in unneeded sem_post() calls,
eventually leading to EOVERFLOW errors being logged, "sem_post() failed
while wake shmtx (75: Value too large for defined data type)".

To close the race, mtx->wait is now decremented if we obtain the lock right
after incrementing it in ngx_shmtx_lock().  The result can become -1 if a
concurrent ngx_shmtx_unlock() decrements mtx->wait before the added code does.
However, that only leads to one extra iteration in the next call of
ngx_shmtx_lock().

src/core/ngx_shmtx.c

diff --git a/src/core/ngx_shmtx.c b/src/core/ngx_shmtx.c
index a62999f..6230dc0 100644
--- a/src/core/ngx_shmtx.c
+++ b/src/core/ngx_shmtx.c
@@ -101,6 +101,7 @@
             (void) ngx_atomic_fetch_add(mtx->wait, 1);
 
             if (*mtx->lock == 0 && ngx_atomic_cmp_set(mtx->lock, 0, ngx_pid)) {
+                (void) ngx_atomic_fetch_add(mtx->wait, -1);
                 return;
             }
 
@@ -174,7 +175,7 @@
 
         wait = *mtx->wait;
 
-        if (wait == 0) {
+        if ((ngx_atomic_int_t) wait <= 0) {
             return;
         }