sdscatfmt call sdsMakeRoomFor, asked for more space than intended (#8286)

instead of asking for the extra new space it wanted, it asked to grow the
string by the size it already has too.
i.e. a string of 1000 bytes, needing to grow by 10 bytes, would have been
asking for an **additional** 1010 bytes.

src/sds.c

@@ -642,7 +642,7 @@ sds sdscatfmt(sds s, char const *fmt, ...) {
     /* To avoid continuous reallocations, let's start with a buffer that
      * can hold at least two times the format string itself. It's not the
      * best heuristic but seems to work in practice. */
-    s = sdsMakeRoomFor(s, initlen + strlen(fmt)*2);
+    s = sdsMakeRoomFor(s, strlen(fmt)*2);
     va_start(ap,fmt);
     f = fmt;    /* Next format specifier byte to process. */
     i = initlen; /* Position of the next byte to write to dest str. */