nspawn-oci: add asserts for UID/GID validity after dispatch

Coverity flags UINT32_MAX - data.container_id as an underflow
when container_id could be UID_INVALID (UINT32_MAX). After
successful sd_json_dispatch_uid_gid(), the values are guaranteed
valid, but Coverity cannot trace through the callback. Add
asserts to document this invariant.

CID#1548072

Follow-up for 91c4d1affd

src/nspawn/nspawn-oci.c

@@ -22,6 +22,7 @@
 #include "string-util.h"
 #include "strv.h"
 #include "time-util.h"
+#include "user-util.h"
 
 /* TODO:
  * OCI runtime tool implementation
@@ -685,6 +686,10 @@ static int oci_uid_gid_mappings(const char *name, sd_json_variant *v, sd_json_di
         if (r < 0)
                 return r;
 
+        /* Silence static analyzers, sd_json_dispatch_uid_gid() already validates */
+        assert(uid_is_valid(data.host_id));
+        assert(uid_is_valid(data.container_id));
+
         if (data.range > UINT32_MAX - data.host_id ||
             data.range > UINT32_MAX - data.container_id)
                 return json_log(v, flags, SYNTHETIC_ERRNO(EINVAL),