Fix -Wsign-compare warning

[Ben Noordhuis]

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Hello, commit 9ce7316 breaks build on e2k arch with lcc 1.23.21 (an EDG-based custom compiler for the target platform):

cc  -I. -DHTTP_PARSER_STRICT=1  -Wall -Wextra -Werror -O0 -g  -c http_parser.c -o http_parser_g.o
lcc: "http_parser.c", line 2520: error: comparison between
          signed and unsigned integer expressions
          [-Werror=sign-compare]
      assert(off + len <= buflen && "Port number overflow");