msxml3: Fix printing NULL strings

[Dan Kegel]

Hi Francois,

your patch has a signedness problem which makes -Werror unhappy.
(We could disable signedness warnings, but since we don't currently
have any, it's nice to keep them out of the tree...)

Here's the log from the buildbot:

$ ccache gcc -c -I. -I. -I../../include -I../../include
-I/usr/include/libxml2   -I/usr/include/libxml2   -D__WINESRC__
-DCOM_NO_WINDOWS_H -D_REENTRANT -fPIC -Wall -pipe -fno-strict-aliasing
-Wdeclaration-after-statement -Wempty-body -Wstrict-prototypes
-Wtype-limits -Wwrite-strings -Wpointer-arith -Wlogical-op  -g -O0
-Werror  -o element.o element.c

cc1: warnings being treated as errors
element.c: In function ‘variant_from_dt’:
element.c:666: error: pointer targets in passing argument 1 of
‘debugstr_a’ differ in signedness
../../include/wine/debug.h:259: note: expected ‘const char *’ but
argument is of type ‘const xmlChar *’
element.c: In function ‘element_get_dt’:
element.c:732: error: pointer targets in passing argument 1 of
‘debugstr_a’ differ in signedness
../../include/wine/debug.h:259: note: expected ‘const char *’ but
argument is of type ‘const xmlChar *’
element.c: In function ‘domelem_put_nodeTypedValue’:
element.c:789: error: pointer targets in passing argument 1 of
‘debugstr_a’ differ in signedness
../../include/wine/debug.h:259: note: expected ‘const char *’ but
argument is of type ‘const xmlChar *’
element.c: In function ‘domelem_put_dataType’:
element.c:937: error: pointer targets in passing argument 1 of
‘debugstr_a’ differ in signedness
../../include/wine/debug.h:259: note: expected ‘const char *’ but
argument is of type ‘const xmlChar *’
make[1]: Leaving directory
`/home/bob/wineslave.dir/sandbox/slave/runtests-default/build/dlls/msxml3'
make[1]: *** [element.o] Error 1
make: *** [dlls/msxml3] Error 2