glad2 static analyzer warnings
From #208 

```
vulkan.c(416): warning C6387: 'extensions[i]' could be '0': this does not adhere to the specification for the function 'memcpy'.
vulkan.c(434): warning C6387: 'extensions[instance_extension_count+i]' could be '0': this does not adhere to the specification for the function 'memcpy'.
vulkan.c(412): warning C6385: Reading invalid data from 'ext_properties': the readable size is 'max_extension_count*sizeof(VkExtensionProperties)' bytes, but '520' bytes may be read.
```

Those should be fixed.