gtk.c: fix compiling failure with option -g -O

There were compiling failure with option -g -O
...
././gtk.c: In function 'configure_area':
././gtk.c:397:2: error: 'cr' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  cairo_set_source_rgb(cr,
  ^
././gtk.c:384:14: note: 'cr' was declared here
     cairo_t *cr;
              ^
././gtk.c: In function 'main':
././gtk.c:2911:6: error: 'error' may be used uninitialized in this function [-Werror=maybe-uninitialized]
      fprintf(stderr, "%s: %s\n", pname, error);
      ^
cc1: all warnings being treated as errors
...

Initialized pointer 'cr' and 'error' with NULL

Upstream-Status: Pending

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
---
 gtk.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/gtk.c b/gtk.c
index a2eba2c..c54bf63 100644
--- a/gtk.c
+++ b/gtk.c
@@ -2481,7 +2481,7 @@ char *fgetline(FILE *fp)
 int main(int argc, char **argv)
 {
     char *pname = argv[0];
-    char *error;
+    char *error = NULL;
     int ngenerate = 0, print = FALSE, px = 1, py = 1;
     int time_generation = FALSE, test_solve = FALSE, list_presets = FALSE;
     int soln = FALSE, colour = FALSE;
-- 
1.9.1