ext: Lookup by key return NULL or ENOENT

Make it possible to distinguish 'no val' from 'empty val'.
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/external.c

+#include "common/err.h"
 #include "common/list.h"
 #include "cr_options.h"
 #include "xmalloc.h"
@@ -36,8 +37,10 @@ char *external_lookup_by_key(char *key)
 			continue;
 		if (ext->id[len] == ':')
 			return ext->id + len + 1;
+		else if (ext->id[len] == '\0')
+			return NULL;
 	}
-	return NULL;
+	return ERR_PTR(-ENOENT);
 }
 
 int external_for_each_type(char *type, int (*cb)(struct external *, void *), void *arg)

criu/mount.c

@@ -484,7 +484,7 @@ static int try_resolve_ext_mount(struct mount_info *info)
 		char *val;
 
 		val = external_lookup_by_key(devstr);
-		if (val) {
+		if (!IS_ERR_OR_NULL(val)) {
 			char *source;
 			int len;
 
@@ -1452,7 +1452,7 @@ static char *resolve_source(struct mount_info *mi)
 		char *val;
 
 		val = external_lookup_by_key(mi->source);
-		if (val)
+		if (!IS_ERR_OR_NULL(val))
 			return val;
 
 		if (!stat(mi->source, &st) && S_ISBLK(st.st_mode) &&