ppd-emit.c: Fix SEGV in 'ppdEmitString()'

[Drawishe]

When using testppd.c as a harness, a fuzzer found a way to call ppdPageSize() with NULL return value. This caused a segmentation fault because the size structure, which is used by values[pos], was assigned a NULL value. To avoid this, we need to add a NULL value check for the size structure, free allocated memory, and return NULL.

Fixes #849

[zdohnal]

Note to myself - push it into libppd once merged.

[zdohnal]

Tests are failing on the similar place as before, so it looks ok to me.

[zdohnal]

@Drawishe thank you for the fix!

[zdohnal]

One late note - it would be great if you squashed the commits next time if possible, it gives cleaner git history.

[jsmeix]

@zdohnal did you really mean "git stash"?

[zdohnal]

@jsmeix thanks Johannes, it was really "squash"....