diff.c: guard config parser from value=NULL

diff.external, diff.*.command, diff.color.*, color.diff.* and
diff.*.funcname configuration variables expect a string value.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -158,15 +158,20 @@ int git_diff_ui_config(const char *var, const char *value)
 		return 0;
 	}
 	if (!strcmp(var, "diff.external")) {
+		if (!value)
+			return config_error_nonbool(var);
 		external_diff_cmd_cfg = xstrdup(value);
 		return 0;
 	}
 	if (!prefixcmp(var, "diff.")) {
 		const char *ep = strrchr(var, '.');
 
 		if (ep != var + 4) {
-			if (!strcmp(ep, ".command"))
+			if (!strcmp(ep, ".command")) {
+				if (!value)
+					return config_error_nonbool(var);
 				return parse_lldiff_command(var, ep, value);
+			}
 		}
 	}
 
@@ -177,15 +182,20 @@ int git_diff_basic_config(const char *var, const char *value)
 {
 	if (!prefixcmp(var, "diff.color.") || !prefixcmp(var, "color.diff.")) {
 		int slot = parse_diff_color_slot(var, 11);
+		if (!value)
+			return config_error_nonbool(var);
 		color_parse(value, var, diff_colors[slot]);
 		return 0;
 	}
 
 	if (!prefixcmp(var, "diff.")) {
 		const char *ep = strrchr(var, '.');
 		if (ep != var + 4) {
-			if (!strcmp(ep, ".funcname"))
+			if (!strcmp(ep, ".funcname")) {
+				if (!value)
+					return config_error_nonbool(var);
 				return parse_funcname_pattern(var, ep, value);
+			}
 		}
 	}