newline \n and carriage return \r problems with MultiFieldReview<'a>

[dj8yfo]

A newline '\n' added to payload behaves differently on different platforms:

1. nanos in speculos emulator (test passes) - newline char is displayed as empty string "" , hidden part of my_field.value is displayed
2. nanos on actual device - newline char is displayed as ?, hidden part of my_field.value is not displayed. this can be potentially used by misbehaving client to hide parts of message from clear signing.
3. nanosp and nanox in speculos - both crash with a panic 0xe000

Same details reiterated in LedgerHQ/app-boilerplate-rust#55

Similar issue was observed with \r.

[dj8yfo]

as one of potential source of problems offset_c may be checked to be of 0-95 range to prevent out of bounds array access.
All chars below 0x20 may be coerced to SPACE with saturating_sub.

Controlling that *c as usize - 0x20 is not greater than 95 is related to #124