cbi-util: Help GCC determine that variable is initialized

GCC 6.3 can't tell that we enforce the variable 'size' being set,
so initialize it to 0 to make the warning go away.  The code does
actually verify that size is set, but not by checking the size
variable itself.

util/cbi-util.c: In function 'main':
util/cbi-util.c:139:8: error: 'size' may be used uninitialized in this
function [-Werror=maybe-uninitialized]
buf = malloc(size);
        ^~~~~~~~~~~~
util/cbi-util.c:233:11: note: 'size' was declared here
uint32_t size;
           ^~~~

BUG=b:72609872
BRANCH=None
TEST=Build with coreboot toolchain.

Change-Id: Ide41a0fce40254f2fa3a8626dec75840a728d967
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://chromium-review.googlesource.com/890703
Commit-Ready: Martin Roth <martinroth@chromium.org>
Tested-by: Martin Roth <martinroth@chromium.org>
Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>
Reviewed-by: Edward Hill <ecgh@chromium.org>
Reviewed-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-by: Jett Rink <jettrink@chromium.org>

util/cbi-util.c

@@ -230,7 +230,7 @@ int main(int argc, char **argv)
 	int mode = OPT_MODE_NONE;
 	const char *cbi_filename = NULL;
 	struct board_info bi;
-	uint32_t size;
+	uint32_t size = 0;
 	uint8_t erase = 0xff;
 	int show_all = 0;
 	int parse_error = 0;