commit: handle large commit messages in utf8 verification

Running t4205 under UBSan with the EXPENSIVE prereq enabled triggers an error when we try to create a commit message that is over 2GB: commit.c:1574:6: runtime error: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int' The problem is that find_invalid_utf8() is not prepared to handle large buffers, as it uses an "int" to represent buffer sizes and offsets. We can fix this with a few changes: 1. We'll take in "len" as a size_t (which is what the caller has anyway, since it's working with a strbuf). 2. We need to return a size_t to give the offset to the invalid utf8, but we also need a sentinel value for "no invalid value" (previously "-1"). Let's split these to return a bool for "found invalid utf8" and then pass back the offset as an out-parameter. We'll switch the function name to match the new semantics. 3. The caller in verify_utf8() uses a "long" to store buffer positions, which is a bit funny. This goes back to 08a94a145c (commit/commit-tree: correct latin1 to utf-8, 2012-06-28) and is perhaps trying to match our use of "unsigned long" for object sizes (though we don't care about it ever becoming negative here). This should be a size_t, too, as some platforms (like Windows) still use a 32-bit long on machines with 64-bit pointers. 4. The "bytes" field within find_invalid_utf() does not have range problems. It is the number of bytes the utf8 sequence claims to have, so is limited by how many bits can be set in a single 8-bit byte. However, if we leave it as an "int" then the compiler will complain about the sign mismatch when comparing it to "len". So let's make it unsigned, too. All of this is a little silly, of course, because 2GB text commit messages are clearly nonsense. So we might consider rejecting them outright, but it is easy enough to make these helper functions more robust in the meantime. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>