patch 8.0.0975: using freed memory when setting 'backspace'

Problem:    Using freed memory when setting 'backspace'.
Solution:   When changing oldval also change origval.
diff --git a/src/option.c b/src/option.c
index 4810641..61d3579 100644
--- a/src/option.c
+++ b/src/option.c
@@ -4927,6 +4927,8 @@
 					break;
 				}
 				vim_free(oldval);
+				if (origval == oldval)
+				    origval = *(char_u **)varp;
 				oldval = *(char_u **)varp;
 			    }
 			    /*