Panic in `find_from` with `mailparse` 0.16.0

[link2xt]

I don't have an example message, but the error is Abort message: 'assertion failed: ix_start < line.len()' which seems to happen at this line:

mailparse/src/lib.rs

Line 123 in d6494b9

assert!(ix_start < line.len());

Downstream Delta Chat issue is chatmail/core#6581

[link2xt]

I have produced a testcase with a fuzzer:

crash.txt

[link2xt]

Simplified the test to:

Content-Type: multipart/mixed; boundary="foobar"

--foobar--

[link2xt]

Made a fix #131