axfer/test: fix uninitialized warning

axfer/test: fix uninitialized warning

This commit fixes uninitialized return value from a call of test_mux() and
text_demux() to suppress warnings below:

$ make check
...
gcc -DHAVE_CONFIG_H -I. -I../../include     -O2 -Wall -pipe -g -MT mapper-test.o -MD -MP -MF .deps/mapper-test.Tpo -c -o mapper-test.o mapper-test.c
mapper-test.c: In function ‘test_mapper’:
mapper-test.c:241:9: warning: ‘err’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  return err;
         ^~~
mapper-test.c:231:5: warning: ‘err’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  if (err < 0)
     ^

Fixes: 39d1ab8a0cb4: ('axfer: add a unit test for mapper interface')
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>