Mapbox: null pointer dereference and segfault in tile-count-merge

This crash was triggered with 642f773 while fuzzing tile-count-merge with AFL on Debian 8 x64. ./tile-count-merge -o /dev/null test000 ASAN:SIGSEGV ================================================================= ==10201==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 pc...