BUILD: mworker: always initialize the saveptr of strtok_r()

BUILD: mworker: always initialize the saveptr of strtok_r()

Building with some libcs which define strtok_r() as an inline function
can yield a possibly uninitialized warning due to a loop dereferencing
this save pointer early, even though the doc clearly mentions that it
is ignored. This is actually more of a mismatch between the compiler
and the libc (gcc-4.7 and glibc-2.23 in that case). It's trivial to
set s2 to NULL here so let's do it to please this old couple. Note
that while the warning is triggered in all supported versions, there's
no point backporting it since it's unlikely this combination will be
relevant outside of backwards compatibility checks now.