getch2.c: fix negative keycodes returned for non-ascii

getch2.c: fix negative keycodes returned for non-ascii

getch2.c read data into a "char" array, and returned values other than
escape sequences directly from there. This meant that it could return
negative values (except on platforms where "char" is unsigned) if the
input contained bytes >= 128. This would break later parsing in
input.c as the values would be interpreted as having the MP_KEY_DOWN
flag set, which would make the key binding code think a key is held
down (and never released). Fix by changing the buffer type to unsigned
char.

The bug itself was very old, but started triggering visible breakage
more easily after commit 82b8f89baea ("input: rework event reading and
command queuing"). Before that the key values would be passed through
the input.c "key read function" interface, which (mis)interpreted the
negative values as errors from the function, and in most cases
discarded them without much visible effect.