sched/pthread: Use -1 instead of 0 as PID for unclaimed mutexes

sched/pthread/pthread_condtimedwait.c

@@ -270,7 +270,7 @@ int pthread_cond_timedwait(FAR pthread_cond_t *cond, FAR pthread_mutex_t *mutex,
                 {
                   /* Give up the mutex */
 
-                  mutex->pid = 0;
+                  mutex->pid = -1;
                   ret = pthread_givesemaphore((sem_t*)&mutex->sem);
                   if (ret)
                     {

sched/pthread/pthread_condwait.c

@@ -113,7 +113,7 @@ int pthread_cond_wait(FAR pthread_cond_t *cond, FAR pthread_mutex_t *mutex)
       sdbg("Give up mutex / take cond\n");
 
       sched_lock();
-      mutex->pid = 0;
+      mutex->pid = -1;
       ret = pthread_givesemaphore((sem_t*)&mutex->sem);
 
       /* Take the semaphore */
@@ -127,7 +127,7 @@ int pthread_cond_wait(FAR pthread_cond_t *cond, FAR pthread_mutex_t *mutex)
       ret |= pthread_takesemaphore((sem_t*)&mutex->sem);
       if (!ret)
         {
-          mutex->pid = getpid();;
+          mutex->pid = getpid();
         }
     }
 

sched/pthread/pthread_mutexdestroy.c

@@ -108,7 +108,7 @@ int pthread_mutex_destroy(FAR pthread_mutex_t *mutex)
 
       /* Is the semaphore available? */
 
-      if (mutex->pid != 0)
+      if (mutex->pid != -1)
         {
           ret = EBUSY;
         }

sched/pthread/pthread_mutexinit.c

@@ -115,7 +115,7 @@ int pthread_mutex_init(FAR pthread_mutex_t *mutex, FAR const pthread_mutexattr_t
 
       /* Indicate that the semaphore is not held by any thread. */
 
-      mutex->pid = 0;
+      mutex->pid = -1;
 
       /* Initialize the mutex like a semaphore with initial count = 1 */
 

sched/pthread/pthread_mutexunlock.c

@@ -148,7 +148,7 @@ int pthread_mutex_unlock(FAR pthread_mutex_t *mutex)
         {
           /* Nullify the pid and lock count then post the semaphore */
 
-          mutex->pid    = 0;
+          mutex->pid    = -1;
 #ifdef CONFIG_MUTEX_TYPES
           mutex->nlocks = 0;
 #endif