src/bootchooser: fix g_data_input_stream_read_line error handling
src/bootchooser: fix g_data_input_stream_read_line error handling

g_data_input_stream_read_line may return NULL but leave the error
argument unset to indicate there was no content to read.

In this case we must not try to propagate an unset error but generate an
appropriate error message ourselves.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

rauc/rauc,rauc/rauc,rauc/rauc,jluebbe/rauc,ejoerns/rauc,ejoerns/rauc,jluebbe/rauc,ejoerns/rauc,jluebbe/rauc