Crash with unsanitized filename input

See below.

```
ragg::agg_png("%s_results.png")
plot(1:10)
dev.off()

 *** caught segfault ***
address 0x0, cause 'memory not mapped'

Traceback:
 1: dev.off()

Possible actions:
1: abort (with core dump, if enabled)
2: normal R exit
3: exit R without saving workspace
4: exit R saving workspace
```

Should be a simple fix, `grDevices::png` doesn't allow this filename. 