[HUST][CSE]mips/common/exception.c/rt_set_except_vector (#7238)

* mips/common/exception.c/rt_set_except_vector

* Update exception.c

---------

Co-authored-by: Bernard Xiong <bernard.xiong@gmail.com>

libcpu/mips/common/exception.c

@@ -82,13 +82,14 @@ exception_func_t sys_exception_handlers[RT_EXCEPTION_MAX];
  */
 exception_func_t rt_set_except_vector(int n, exception_func_t func)
 {
-    exception_func_t old_handler = sys_exception_handlers[n];
+    exception_func_t old_handler;
 
-    if ((n == 0) || (n > RT_EXCEPTION_MAX) || (!func))
+    if ((n < 0) || (n >= RT_EXCEPTION_MAX) || (!func))
     {
         return 0;
     }
 
+    old_handler = sys_exception_handlers[n];
     sys_exception_handlers[n] = func;
 
     return old_handler;