Fail Executable_ulfex upon duplicate keys in malformed input

...instead of causing use-after-free of pMergeEntrys, which would be destroyed
during the (non-adding) emplace call but would still be used in the following
if/else block (see the commit message of
c6e2052b6f "Update git submodules: Fix duplicate
key typo")

Change-Id: Iac8d67e61aba0144d3d5807f478c7b330d7c4c81
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/130235
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

l10ntools/source/merge.cxx

@@ -21,7 +21,9 @@
 #include <sal/log.hxx>
 
 #include <algorithm>
+#include <cstdlib>
 #include <fstream>
+#include <iostream>
 #include <string>
 #include <vector>
 
@@ -298,7 +300,11 @@ void MergeDataFile::InsertEntry(
     if( !pMergeEntrys )
     {
         pMergeEntrys = new MergeEntrys;
-        aMap.emplace( sKey, std::unique_ptr<MergeEntrys>(pMergeEntrys) );
+        if (!aMap.emplace( sKey, std::unique_ptr<MergeEntrys>(pMergeEntrys) ).second)
+        {
+            std::cerr << "Duplicate entry " << sKey << "\n";
+            std::exit(EXIT_FAILURE);
+        }
     }