Subcorpus, crash on delete with subcorpus not correctly computed

To reproduce :
1- create a concordance of “ []”
2- delete the “” elements in XML-TXM files
3- update the corpus
4- TXM tries to rebuild the concordance
5- delete the concordance ->CRASH

#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00007f0ec70df108, pid=18491, tid=21508
#
# JRE version: OpenJDK Runtime Environment Temurin-17.0.4.1+1 (17.0.4.1+1) (build 17.0.4.1+1)
# Java VM: OpenJDK 64-Bit Server VM Temurin-17.0.4.1+1 (17.0.4.1+1, mixed mode, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# C  [libcqpjni.so+0x51108]  Java_org_txm_searchengine_cqp_MemCqiServer_dropSubCorpus+0xf8

(from redmine: issue id 3406, created on 2023/06/20 by Matthieu Decorde)