#1 %s formatstring used for NULL ptrs

[Jan Kratochvil]

Currently the whole project may pass NULL char *
as argument for "%s" in formatstring.
This feature works in Linux but it is not crossplatform,
for example it will SIGSEGV on Solaris.
Due to this fact is on Solaris and similiar NULL-sensitive
platforms --enable-debug of GGN library completely
unusable.

Checks should be done for all the places where NULL can
be passed to "%s" and some workaround should be implemented,
like: (string?string:"(null)")
Macro for such functionality preferred.