avoid non init'ed im if no png support

examples/crop.c

@@ -33,6 +33,7 @@ gdImagePtr read_png(const char *filename)
 #ifdef HAVE_LIBPNG
 	im = gdImageCreateFromPng(fp);
 #else
+	im = NULL;
 	printf("No PNG support. Cannot read image.\n");
 #endif
 	fclose(fp);

src/pngtogd2.c

@@ -33,6 +33,7 @@ main (int argc, char **argv)
 #ifdef HAVE_LIBPNG
 	im = gdImageCreateFromPng (in);
 #else
+	im = NULL;
 	fprintf (stderr, "No PNG library support available.\n");
 #endif
 	fclose (in);