target/image: fix undefined behaviour when loading with GDB

The image struct is malloc'd and hence base_address_set doesn't have a
defined value.

Caught by Valgrind.

Change-Id: Ice15b2299fc768e44e8034eeb93e035076eacd03
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2524
Tested-by: jenkins
Reviewed-by: Stian Skjelstad <stian@nixia.no>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/target/image.c

@@ -788,6 +788,7 @@ int image_open(struct image *image, const char *url, const char *type_string)
 		}
 	} else if (image->type == IMAGE_BUILDER) {
 		image->num_sections = 0;
+		image->base_address_set = 0;
 		image->sections = NULL;
 		image->type_private = NULL;
 	}