curl: Heap-buffer-overflow (Out-of-Bounds Read) in DoH hostname encoding

Summary: I found a heap-buffer-overflow in the dohreqencode function in lib/doh.c. The bug happens when curl processes a DNS-over-HTTPS request for a hostname that is an empty string. The code gets the string length as 0, then tries to access hostlen - 1, which becomes host-1. This is an...