error.c: protect mrb_print_error() from exceptions.

Now the function can be called when `mrb->jmp` is not set.

src/error.c

@@ -630,6 +630,26 @@ mrb_raise_nomemory(mrb_state *mrb)
   }
 }
 
+MRB_API void
+mrb_print_error(mrb_state *mrb)
+{
+#ifndef MRB_NO_STDIO
+  if (mrb->jmp == NULL) {
+    struct mrb_jmpbuf c_jmp;
+    MRB_TRY(&c_jmp) {
+      mrb->jmp = &c_jmp;
+      mrb_print_backtrace(mrb);
+    } MRB_CATCH(&c_jmp) {
+      /* ignore exception during print_backtrace() */
+    } MRB_END_EXC(&c_jmp);
+    mrb->jmp = NULL;
+  }
+  else {
+    mrb_print_backtrace(mrb);
+  }
+#endif
+}
+
 void
 mrb_init_exception(mrb_state *mrb)
 {

src/print.c

@@ -49,12 +49,6 @@ mrb_p(mrb_state *mrb, mrb_value obj)
 }
 
 
-MRB_API void
-mrb_print_error(mrb_state *mrb)
-{
-  mrb_print_backtrace(mrb);
-}
-
 MRB_API void
 mrb_show_version(mrb_state *mrb)
 {
@@ -78,11 +72,6 @@ mrb_p(mrb_state *mrb, mrb_value obj)
 {
 }
 
-MRB_API void
-mrb_print_error(mrb_state *mrb)
-{
-}
-
 MRB_API void
 mrb_show_version(mrb_state *mrb)
 {