replace assert with mrb_assert

src/numeric.c

@@ -4,7 +4,6 @@
 ** See Copyright Notice in mruby.h
 */
 
-#include <assert.h>
 #include <float.h>
 #if defined(__FreeBSD__) && __FreeBSD__ < 4
 # include <floatingpoint.h>
@@ -782,7 +781,7 @@ fixdivmod(mrb_state *mrb, mrb_int x, mrb_int y, mrb_int *divp, mrb_int *modp)
 {
   mrb_int div, mod;
 
-  /* TODO: add assert(y != 0) to make sure */
+  /* TODO: add mrb_assert(y != 0) to make sure */
 
   if (y < 0) {
     if (x < 0)