r3.showdspf: fix null pointer dereference

In the current execution, when we face an error while trying
to open a database file, as part of error log we are dereferencing
a NULL file pointer (which is returend when we try to open a file).
Ideally we should be printing the name of the database file rather
than the file pointer. This fixes the problem of null pointer
dereference. Found via cppcheck tool.

Signed-off-by: Mohan Yelugoti <ymdatta.work@gmail.com>

raster3d/r3.showdspf/main_ogl.c

@@ -202,8 +202,7 @@ int main(int argc, char **argv)
             G_fatal_error(buff);
         }
         if ((Headfax.dspfinfp = G_fopen_old(buff, dsp, mapset)) == NULL) {
-            fprintf(stderr, "Unable to open <%s> for reading\n",
-                    Headfax.dspfinfp);
+            fprintf(stderr, "Unable to open <%s> for reading\n", dsp);
             exit(EXIT_FAILURE);
         }