Make the error checking more robust

Author: kulikjak

Modules/posixmodule.c

@@ -7794,8 +7794,10 @@ os_sched_get_priority_max_impl(PyObject *module, int policy)
 {
     int max;
 
+    /* make sure that errno is cleared before the call */
+    errno = 0;
     max = sched_get_priority_max(policy);
-    if (max == -1)
+    if (max == -1 && errno)
         return posix_error();
     return PyLong_FromLong(max);
 }
@@ -7813,8 +7815,12 @@ static PyObject *
 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 == -1)
+    int min;
+
+    /* make sure that errno is cleared before the call */
+    errno = 0;
+    min = sched_get_priority_min(policy);
+    if (min == -1 && errno)
         return posix_error();
     return PyLong_FromLong(min);
 }