cmd: nvedit: env_get_f must check for env_get_char error codes

env_get_f calls env_get_char to load single characters from the
environment. However, the return value of env_get_char was not
checked for errors. Now if the env driver does not support the
.get_char call, env_get_f did not notice this and looped over the
whole size of the environment, calling env_get_char over 8000
times with the default settings, just to return an error in the
end.

Fix this by checking if env_get_char returns < 0.

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>