show_stats(): fix stats width calculation

Before this patch, name_width becomes negative or null for width values
less than 15 and name_width values greater than 25 (default: 50). This
leads to output random data.

This patch checks for minimal width and name_width values.
Signed-off-by: NOlivier Marin <dkr@freesurf.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

diff.c

@@ -830,12 +830,12 @@ static void show_stats(struct diffstat_t* data, struct diff_options *options)
 	/* Sanity: give at least 5 columns to the graph,
 	 * but leave at least 10 columns for the name.
 	 */
-	if (width < name_width + 15) {
-		if (name_width <= 25)
-			width = name_width + 15;
-		else
-			name_width = width - 15;
-	}
+	if (width < 25)
+		width = 25;
+	if (name_width < 10)
+		name_width = 10;
+	else if (width < name_width + 15)
+		name_width = width - 15;
 
 	/* Find the longest filename and max number of changes */
 	reset = diff_get_color_opt(options, DIFF_RESET);