string-util: Prevent infinite loop pegging CPU on malformed ESC input

string-util: Prevent infinite loop pegging CPU on malformed ESC input

string_has_ansi_sequence() currently does this to look for ESC input:

    t = memchr(s, 0x1B, ...)

So each iteration re-searches from the original start pointer. But if we
find an ESC byte that does *not* start a valid ANSI sequence (like "\x1B
", or an ESC at the end of the string), then ansi_sequence_length()
returns 0, and if that ESC is still in the search window, we will just
spin consuming 100% CPU forever.

Fix this by always advancing past rejected ESC bytes.