remote-ext: do not segfault for blank lines

remote-ext: do not segfault for blank lines

Instead of stripping space characters past the beginning of the
line and overflowing a buffer, stop at the beginning of the line
(mimicking the corresponding fix in remote-fd).

The argument to isspace does not need to be cast explicitly because
git isspace takes care of that already.

Noticed-by: Junio C Hamano <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>