_sdbm.c: fix pointers to shift

* ext/sdbm/_sdbm.c (delpair): dst and src point at the ends of
  each data, whereas memmove requires pointers to the beginnings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49494 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/sdbm/_sdbm.c

@@ -802,7 +802,7 @@ delpair(char *pag, datum key)
 		}
 #else
 #ifdef MEMMOVE
-		memmove(dst, src, m);
+		memmove(dst-m, src-m, m);
 #else
 		while (m--)
 			*--dst = *--src;