tests: Fix a stack buffer overflow
tests: Fix a stack buffer overflow

When using the length argument for scanf's %s formatting option, the
target buffer needs to have space for the terminating null character as
well.

This fixes the first part of #551. The underflow, however, is still
there.

rastislavszabo/sysrepo,lukasmacko/sysrepo,lukasmacko/sysrepo,lukasmacko/sysrepo,fanchanghu/sysrepo,rastislavszabo/sysrepo,lukasmacko/sysrepo,fanchanghu/sysrepo,rastislavszabo/sysrepo,lukasmacko/sysrepo,morganzhh/sysrepo,fanchanghu/sysrepo,rastislavszabo/sysrepo,fanchanghu/sysrepo,morganzhh/sysrepo,fanchanghu/sysrepo,fanchanghu/sysrepo,morganzhh/sysrepo,rastislavszabo/sysrepo,fanchanghu/sysrepo,rastislavszabo/sysrepo,lukasmacko/sysrepo,rastislavszabo/sysrepo,morganzhh/sysrepo,morganzhh/sysrepo,morganzhh/sysrepo,lukasmacko/sysrepo,morganzhh/sysrepo