core: refactor check for valid pid

move into header and remove redundant condition

core/include/kernel_types.h

@@ -67,6 +67,18 @@
  */
 typedef int16_t kernel_pid_t;
 
+/**
+ * @brief   Determine if the given pid is valid
+ *
+ * @param[in]   pid     The pid to check
+ *
+ * @return      true if the pid is valid, false otherwise
+ */
+static inline int pid_is_valid(kernel_pid_t pid)
+{
+    return ((KERNEL_PID_FIRST <= pid) && (pid <= KERNEL_PID_LAST));
+}
+
 #ifdef __cplusplus
 }
 #endif

core/thread.c

@@ -34,7 +34,7 @@
 
 volatile tcb_t *thread_get(kernel_pid_t pid)
 {
-    if ((pid != KERNEL_PID_UNDEF) && (KERNEL_PID_FIRST <= pid) && (pid <= KERNEL_PID_LAST)) {
+    if (pid_is_valid(pid)) {
         return sched_threads[pid];
     }
     return NULL;