qsort: add some forgotten ()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/qsort.h

@@ -25,7 +25,7 @@
     void *stack[64][2];\
     int sp= 1;\
     stack[0][0] = p;\
-    stack[0][1] = p+num-1;\
+    stack[0][1] = (p)+(num)-1;\
     while(sp){\
         type *start= stack[--sp][0];\
         type *end  = stack[  sp][1];\