Dangerous variable-length array (VLA) declaration found with Clang Static Analyzer

Logic error	Dangerous variable-length array (VLA) declaration	main.c	main	line 550, 391, 550, 391

```
unsigned cs_size = 0;
cs_size = img_opt.checksum_size;
unsigned char checksum[cs_size];
```

is ***BAD***! it is better to use:
```
unsigned char *checksum = NULL;
checksum = (unsigned char *)malloc(cs_size);
```
then remember to ```free``` it