Add `mrb_get_arg1()` that retrieves single (and only) argument.

`mrb_get_arg1()` raises `ArgumentError` if the method does not receive one
argument.

And replaces all `mrb_get_args(mrb, "o", &arg)` by the new function.

1 files changed, 1 insertions, 2 deletions

diff --git a/src/error.c b/src/error.c
index d1bd52d41..5d17209e5 100644
--- a/src/error.c
+++ b/src/error.c
@@ -160,9 +160,8 @@ set_backtrace(mrb_state *mrb, mrb_value exc, mrb_value backtrace)
 static mrb_value
 exc_set_backtrace(mrb_state *mrb, mrb_value exc)
 {
-  mrb_value backtrace;
+  mrb_value backtrace = mrb_get_arg1(mrb);
 
-  mrb_get_args(mrb, "o", &backtrace);
   set_backtrace(mrb, exc, backtrace);
   return backtrace;
 }