Replace dm_snprintf with strncpy

Replace dm_snprintf with strncpy

My previous patch fixed incorrect error check for dm_snprintf.
However in this particular case - dm_snprintf has been used differently -
just like strncpy + setting last char with '\0' - so the code had to return
error - because the buffer was to short for whole string.

Patch replaces it with real strncpy.
Also test for alloca() failure is removed - as the program behaviour
is rather undefined in this case - it never returns NULL.