merge-recursive.c: guard config parser from value=NULL

merge.default, merge.*.{name,driver} expect a string value

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

merge-recursive.c

@@ -844,7 +844,8 @@ static int read_merge_config(const char *var, const char *value)
 	int namelen;
 
 	if (!strcmp(var, "merge.default")) {
-		if (value)
+		if (!value)
+			return config_error_nonbool(var);
 		default_ll_merge = strdup(value);
 		return 0;
 	}
@@ -878,14 +879,14 @@ static int read_merge_config(const char *var, const char *value)
 
 	if (!strcmp("name", ep)) {
 		if (!value)
-			return error("%s: lacks value", var);
+			return config_error_nonbool(var);
 		fn->description = strdup(value);
 		return 0;
 	}
 
 	if (!strcmp("driver", ep)) {
 		if (!value)
-			return error("%s: lacks value", var);
+			return config_error_nonbool(var);
 		/*
 		 * merge.<name>.driver specifies the command line:
 		 *
@@ -908,7 +909,7 @@ static int read_merge_config(const char *var, const char *value)
 
 	if (!strcmp("recursive", ep)) {
 		if (!value)
-			return error("%s: lacks value", var);
+			return config_error_nonbool(var);
 		fn->recursive = strdup(value);
 		return 0;
 	}