fix overflow in URL parsing

[DavidKorczynski]

The fuzzer added in #102 finds an overflow in the URL parsing logic, with the following sanitizer report:

==13==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x5020001398f4 at pc 0x557be604430c bp 0x7ffe725a0400 sp 0x7ffe725a03f8
READ of size 1 at 0x5020001398f4 thread T0                       
SCARINESS: 12 (1-byte-read-heap-buffer-overflow)                 
    #0 0x557be604430b in http_parse_host /src/http-parser/./http_parser/http_parser.c:2015:58
    #1 0x557be604430b in http_parser_parse_url /src/http-parser/./http_parser/http_parser.c:2150:9
    #2 0x557be6039317 in LLVMFuzzerTestOneInput /src/fuzzer.c:19:3

This fixes it by ensuring the http_parse_host function does not beyond the buffer it uses for reading.