fix type of mrb_exc_new()'s len parameter

mrb_str_new() takes size_t, so should mrb_exc_new().

include/mruby.h

@@ -365,7 +365,7 @@ MRB_API mrb_value mrb_obj_clone(mrb_state *mrb, mrb_value self);
 #define TOLOWER(c) (ISASCII(c) ? tolower((int)(unsigned char)(c)) : (c))
 #endif
 
-MRB_API mrb_value mrb_exc_new(mrb_state *mrb, struct RClass *c, const char *ptr, long len);
+MRB_API mrb_value mrb_exc_new(mrb_state *mrb, struct RClass *c, const char *ptr, size_t len);
 MRB_API mrb_noreturn void mrb_exc_raise(mrb_state *mrb, mrb_value exc);
 
 MRB_API mrb_noreturn void mrb_raise(mrb_state *mrb, struct RClass *c, const char *msg);

src/error.c

@@ -19,7 +19,7 @@
 #include "mrb_throw.h"
 
 MRB_API mrb_value
-mrb_exc_new(mrb_state *mrb, struct RClass *c, const char *ptr, long len)
+mrb_exc_new(mrb_state *mrb, struct RClass *c, const char *ptr, size_t len)
 {
   mrb_value arg = mrb_str_new(mrb, ptr, len);
   return mrb_obj_new(mrb, c, 1, &arg);