vcfcreatemulti: Avoid segfault when passed VCF file with no variants

Erik;
vcfcreatemulti will currently segfault when passed a VCF file with no variants. I ran into this case when merging files in smaller regions where some had no variants. This small fix correctly outputs an equivalent empty file.

Apologies, my vcflib history is a bit wacky because I have another outstanding pull request. Feel free to manually apply or cherry pick the relevant commit (https://github.com/chapmanb/vcflib/commit/c6fe008a2ac04540ddffb766c13d84aaa08013de).