bpo-41839: Fix error checking in sched_get_priority_ functions

Author: kulikjak

Modules/posixmodule.c

@@ -7795,7 +7795,7 @@ os_sched_get_priority_max_impl(PyObject *module, int policy)
     int max;
 
     max = sched_get_priority_max(policy);
-    if (max < 0)
+    if (max == -1)
         return posix_error();
     return PyLong_FromLong(max);
 }
@@ -7814,7 +7814,7 @@ os_sched_get_priority_min_impl(PyObject *module, int policy)
 /*[clinic end generated code: output=7595c1138cc47a6d input=21bc8fa0d70983bf]*/
 {
     int min = sched_get_priority_min(policy);
-    if (min < 0)
+    if (max == -1)
         return posix_error();
     return PyLong_FromLong(min);
 }