pthread: fix error code checks (thanks, Andreas!).

Most pthread functions return 0 on success and non-zero on error, but those
errors might be positive or negative, so checking for return values in the
Unix style, where errors are less than zero, is a bug.

Fixes Bugzilla libsdl-org#4039.

src/thread/pthread/SDL_syscond.c

@@ -42,7 +42,7 @@ SDL_CreateCond(void)
 
     cond = (SDL_cond *) SDL_malloc(sizeof(SDL_cond));
     if (cond) {
-        if (pthread_cond_init(&cond->cond, NULL) < 0) {
+        if (pthread_cond_init(&cond->cond, NULL) != 0) {
             SDL_SetError("pthread_cond_init() failed");
             SDL_free(cond);
             cond = NULL;

src/thread/pthread/SDL_sysmutex.c

@@ -105,7 +105,7 @@ SDL_LockMutex(SDL_mutex * mutex)
         }
     }
 #else
-    if (pthread_mutex_lock(&mutex->id) < 0) {
+    if (pthread_mutex_lock(&mutex->id) != 0) {
         return SDL_SetError("pthread_mutex_lock() failed");
     }
 #endif
@@ -181,7 +181,7 @@ SDL_UnlockMutex(SDL_mutex * mutex)
     }
 
 #else
-    if (pthread_mutex_unlock(&mutex->id) < 0) {
+    if (pthread_mutex_unlock(&mutex->id) != 0) {
         return SDL_SetError("pthread_mutex_unlock() failed");
     }
 #endif /* FAKE_RECURSIVE_MUTEX */

src/thread/pthread/SDL_systhread.c

@@ -212,7 +212,7 @@ SDL_SYS_SetThreadPriority(SDL_ThreadPriority priority)
     int policy;
     pthread_t thread = pthread_self();
 
-    if (pthread_getschedparam(thread, &policy, &sched) < 0) {
+    if (pthread_getschedparam(thread, &policy, &sched) != 0) {
         return SDL_SetError("pthread_getschedparam() failed");
     }
     if (priority == SDL_THREAD_PRIORITY_LOW) {
@@ -224,7 +224,7 @@ SDL_SYS_SetThreadPriority(SDL_ThreadPriority priority)
         int max_priority = sched_get_priority_max(policy);
         sched.sched_priority = (min_priority + (max_priority - min_priority) / 2);
     }
-    if (pthread_setschedparam(thread, policy, &sched) < 0) {
+    if (pthread_setschedparam(thread, policy, &sched) != 0) {
         return SDL_SetError("pthread_setschedparam() failed");
     }
     return 0;