axfer/test: fix uninitialized warning

axfer/test: fix uninitialized warning

This commit fixes uninitialized return value from a call of test_vector()
to suppress a warning below.

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_vector’:
mapper-test.c:293:6: warning: ‘err’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  int err;
      ^~~

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>