Avoid a compilation warning in w32.c

* src/w32.c (logon_network_drive): Avoid compilation warning
about strncpy arguments.
1 job for master in 3 minutes and 48 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #3108
test-all

00:03:48

 
Name Stage Failure
failed
test-all Test
Makefile:402: recipe for target 'xdisp.o' failed
make[2]: *** [xdisp.o] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/src'
Makefile:424: recipe for target 'src' failed
make[1]: Leaving directory '/builds/emacs/emacs'
make[1]: *** [src] Error 2
make: *** [bootstrap] Error 2
Makefile:1117: recipe for target 'bootstrap' failed
ERROR: Job failed: exit code 1