Lock key during rpmGetSubkeys()

This should be thread save as it still is in the public API. The
internal use would be fine otherwise. So the lock can be removed if this
gets made internal only.

Resolves: rpm-software-management#3351

rpmio/rpmkeyring.cc

@@ -179,7 +179,12 @@ rpmPubkey *rpmGetSubkeys(rpmPubkey primarykey, int *count)
     int pgpsubkeysCount = 0;
     int i;
 
-    if (primarykey && !pgpPrtParamsSubkeys(primarykey->pkt.data(), primarykey->pkt.size(),
+    if (!primarykey)
+	return NULL;
+
+    rdlock lock(primarykey->mutex);
+
+    if (!pgpPrtParamsSubkeys(primarykey->pkt.data(), primarykey->pkt.size(),
 	    primarykey->pgpkey, &pgpsubkeys, &pgpsubkeysCount)) {
 	/* Returned to C, can't use new */
 	subkeys = (rpmPubkey *)xmalloc(pgpsubkeysCount * sizeof(*subkeys));