Don't worry about the sleuthing: it is what I like doing.  If not, I would have given up fixing bugs in nano a long time ago.  :)  Also, it helped discover a related bug (bug #59855), which I probably would not have found if you had given a clearer reproduction recipe.  So, it's all good.

(After you mentioned that the problem occurred after multiple spell checks, I can now reproduce the sanitizer output by doing two spell checks before exiting -- when going back a few commits.  In current git, things are fine.)

(I will close the bug when the fix is released, in a few days.)

I think commit bc368133 fixes the leak.

perhaps "crash" is imprecise. lsan processes leaks when exiting, so I had finished editing, saved, then exited, and that's when lsan fired. it forces non zero exit.

i can see if enabling nano debug mode is feasible ...

Hmm.  I can produce a crash when using --speller="".

==31740== Invalid write of size 8
==31740==    at 0x12C53A: construct_argument_list (text.c:2043)
==31740==    by 0x12C6FD: treat (text.c:2121)
==31740==    by 0x12D3DD: do_spell (text.c:2537)
==31740==    by 0x121B8F: do_statusbar_input (prompt.c:355)
==31740==    by 0x122162: acquire_an_answer (prompt.c:465)
==31740==    by 0x122574: do_prompt (prompt.c:608)
==31740==    by 0x115E39: do_insertfile (files.c:1150)
==31740==    by 0x120F6B: process_a_keystroke (nano.c:1625)
==31740==    by 0x10E0EF: main (nano.c:2544)
==31740==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

I cannot reproduce, not even after configuring nano with your ./configure command and rebuilding.

From the backtrace, it seems the crash occurred when you initiated a spell check (^T ^S).  But valgrind does not find any memory leak when I run a spell check on the README file (with --speller "aspell -x -c -l en" and nano configured by default).

What is 'set speller' set to in your .nanorc?  What kind of file were you spell checking?

$ nano --version
 GNU nano from git, v5.4-62-gc53da9aa5bff
 (C) 1999-2011, 2013-2020 Free Software Foundation, Inc.
 (C) 2014-2020 the contributors to nano
 Compiled options: --disable-justify --enable-utf8

==26607==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 13 byte(s) in 1 object(s) allocated from:
    #0 0x7f9d288600e8 in __interceptor_malloc /var/tmp/portage/sys-devel/gcc-9.3.0-r2/work/gcc-9.3.0/libsanitizer/asan/asan_malloc_linux.cc:144
    #1 0x55641eafa575 in nmalloc /usr/local/src/nano/src/utils.c:295
    #2 0x55641eafa692 in measured_copy /usr/local/src/nano/src/utils.c:337
    #3 0x55641eafa731 in copy_of /usr/local/src/nano/src/utils.c:348
    #4 0x55641eaf0e97 in construct_argument_list /usr/local/src/nano/src/text.c:2033
    #5 0x55641eaf17e0 in treat /usr/local/src/nano/src/text.c:2121
    #6 0x55641eaf4693 in do_spell /usr/local/src/nano/src/text.c:2537
    #7 0x55641eacec21 in do_statusbar_input /usr/local/src/nano/src/prompt.c:355
    #8 0x55641eacf909 in acquire_an_answer /usr/local/src/nano/src/prompt.c:465
    #9 0x55641ead0878 in do_prompt /usr/local/src/nano/src/prompt.c:608
    #10 0x55641eaa2fe9 in do_insertfile /usr/local/src/nano/src/files.c:1150
    #11 0x55641eaa3e21 in do_execute /usr/local/src/nano/src/files.c:1312
    #12 0x55641eac64ac in process_a_keystroke /usr/local/src/nano/src/nano.c:1625
    #13 0x55641eaccaa6 in main /usr/local/src/nano/src/nano.c:2544
    #14 0x7f9d284fbcfd in __libc_start_main ../csu/libc-start.c:308