patch 8.2.0913: code for resetting v:register is duplicated

Problem:    Code for resetting v:register is duplicated.
Solution:   Add reset_reg_var().
diff --git a/src/proto/evalvars.pro b/src/proto/evalvars.pro
index 0d2e7b4..97641b5 100644
--- a/src/proto/evalvars.pro
+++ b/src/proto/evalvars.pro
@@ -46,6 +46,7 @@
 void set_vim_var_list(int idx, list_T *val);
 void set_vim_var_dict(int idx, dict_T *val);
 void set_argv_var(char **argv, int argc);
+void reset_reg_var(void);
 void set_reg_var(int c);
 char_u *v_exception(char_u *oldval);
 char_u *v_throwpoint(char_u *oldval);