Out of Bounds read in valkey-benchmark (#4142)

Running:
```sh
./valkey-benchmark -t get 10000
```

causes `valkey-benchmark` to segfault.

After option parsing, the remaining argument (`10000`) is interpreted as
a repeat count for an arbitrary command sequence. Since no command
follows the repeat count, `start` becomes equal to `argc`.

When `start` becomes equal to `argc`, the parser evaluates
`sds_args[i][0]` before checking that `i` is still within bounds,
resulting in an out-of-bounds read and undefined behaviour.

This change guards the repeat-count parsing by checking `i < argc`
before accessing `sds_args[i]`.

Note: This avoids reading out of bounds but still does not handle what
happens after that.

Signed-off-by: lightsigma96 <8f34yashjadhav@gmail.com>

Author: lightsigma96

src/valkey-benchmark.c

@@ -2513,7 +2513,7 @@ int main(int argc, char **argv) {
         int seq_len = 0; /* Total number of commands in the sequence. */
         sds cmd_seq = sdsempty();
         for (i = 0; i <= argc; i++) {
-            if (i == start && sds_args[i][0] >= '1' && sds_args[i][0] <= '9') {
+            if (i < argc && i == start && sds_args[i][0] >= '1' && sds_args[i][0] <= '9') {
                 /* Command prefixed by number means repeat command N times. */
                 repeat = atoi(sds_args[i]);
                 start++;