Increase buffer size and prevent write beyond buffer end

Increase buffer size and prevent write beyond buffer end

wpa_config_write_key_mgmt has a buffer size of 50. This is not enough
to fit the longest case. I used a network with "WPA-PSK WPA-EAP
WPA-NONE" and CONFIG_IEEE80211R=y + CONFIG_IEEE80211W=y to produce
a string longer than 50 chars. Increase the buffer size to 100 to
prevent truncated output.

Truncated output is not the only problem. If the buffer end is
reached when adding certain key mgmt types the function does not
return immediately. This leaves pos > end. When a second os_sprintf
is called the calculation of end - pos yields a large positive
number for buffer size. End result is a write beyond the buffer end.
Fix this by bailing out if buffer end is reached.

Signed-hostap: Pontus Fuchs <pontus.fuchs@gmail.com>