SWarp crashes if multithreaded when processing a large number of files

[hbouy]

When swarping a large number of MEF (adding to several tens of thousands of individual extensions) swarp crashes if NTHREADS>1 but works if NTHREADS=1.
This is swarp version 2.41.1 running on CentOS 7.5

The error message is:
-------------- Co-adding frames
Maximum overlap density: 961 frames
*** Error in swarp': double free or corruption (!prev): 0x000000007da4d6b0 *** *** Error in swarp': double free or corruption (!prev): 0x000000007da4d6b0 ***
======= Backtrace: =========
======= Backtrace: =========
/lib64/libc.so.6(+0x81609)[0x7f53ef199609]
/lib64/libc.so.6swarp[0x4bd384]
/lib64/libc.so.6(+0x36340)[0x7f53ef14e340]
/lib64/libc.so.6(+0x36340)[0x7f53ef14e340]
/lib64/libpthread.so.0(+0x7dd5)[0x7f53ef702dd5]
/lib64/libpthread.so.0(+0x7dd5)[0x7f53ef702dd5]
/lib64/libc.so.6(clone+0x6d)[0x7f53ef21602d]
======= Memory map: ========
00400000-005c0000 r-xp 00000000 fd:00 2764376 /usr/bin/swarp
007c0000-007c2000 r--p 001c0000 fd:00 2764376 /usr/bin/swarp
007c2000-007e3000 rw-p 001c2000 fd:00 2764376 /usr/bin/swarp
007e3000-08ba0000 rw-p 00000000 00:00 0
094fc000-fded1000 rw-p 00000000 00:00 0 [heap]