Objective

Fix an invalid read in convert_env_to_header_name (middleware.c:140).

==77960== Invalid read of size 1
==77960==    at 0x204630: convert_env_to_header_name (middleware.c:140)
==77960==    by 0x204630: handle_header_env_key (middleware.c:406)
==77960==    by 0x2002B7: h2o_mruby_iterate_header_values (mruby.c:1151)
==77960==    by 0x2056DD: create_subreq (middleware.c:598)
==77960==    by 0x2056DD: middleware_request_method (middleware.c:791)
... snip ...

Cause

We forward the name pointer by KEY_PREFIX_LEN and loop from there to copy byte-by-byte into the newly created h2o_iovec_t. The issue is that len remains the length of the string with HTTP_ prefix.

Fix

Subtract len by KEY_PREFIX_LEN so that we don't over iterate. The case of len < KEY_PREFIX_LEN is handled earlier in the function. As a result, Valgrind no longer reports an invalid read.