Fix bug in sdscatfmt when % is the last format char (#9173)

For the sdscatfmt function in sds.c, when the parameter fmt ended up with '%',
the behavior is undefined. This commit fix this bug.

Co-authored-by: stafuc <stafuc@gmail.com>

src/sds.c

@@ -682,6 +682,7 @@ sds sdscatfmt(sds s, char const *fmt, ...) {
         switch(*f) {
         case '%':
             next = *(f+1);
+            if (next == '\0') break;
             f++;
             switch(next) {
             case 's':