fix: tform: potential crash due to race when MODMAX/MODMIN

[jodavies]

A worker could free a dollar variable's data, while another worker is trying to read it. Lock the dollar variable before working out its RHS in case the RHS depends on the dollar variable that is being assigned.

This closes #147 and #796 , though #796 contains more discussion for the future.

[tueda]

I wonder if the CI failure might be related to awalsh128/cache-apt-pkgs-action#187. When I print the Valgrind errors, it seems like it could be a libc6-dbg incompatibility(?):

==4640== Memcheck, a memory error detector
==4640== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==4640== Using Valgrind-3.22.0 and LibVEX; rerun with -h for copyright info
==4640== Command: /home/runner/work/form/form/sources/vorm /tmp/form_check_20260213-4633-ehml65/ver_20260213-4633-uya517/ver.frm
==4640== 

valgrind:  Fatal error at startup: a function redirection
valgrind:  which is mandatory for this platform-tool combination
valgrind:  cannot be set up.  Details of the redirection are:
valgrind:  
valgrind:  A must-be-redirected function
valgrind:  whose name matches the pattern:      strlen
valgrind:  in an object with soname matching:   ld-linux-x86-64.so.2
valgrind:  was not found whilst processing
valgrind:  symbols from the object with soname: ld-linux-x86-64.so.2
valgrind:  
valgrind:  Possible fixes: (1, short term): install glibc's debuginfo
valgrind:  package on this machine.  (2, longer term): ask the packagers
valgrind:  for your Linux distribution to please in future ship a non-
valgrind:  stripped ld.so (or whatever the dynamic linker .so is called)
valgrind:  that exports the above-named function using the standard
valgrind:  calling conventions for this platform.  The package you need
valgrind:  to install for fix (1) is called
valgrind:  
valgrind:    On Debian, Ubuntu:                 libc6-dbg
valgrind:    On SuSE, openSuSE, Fedora, RHEL:   glibc-debuginfo
valgrind:  
valgrind:  Note that if you are debugging a 32 bit process on a
valgrind:  64 bit system, you will need a corresponding 32 bit debuginfo
valgrind:  package (e.g. libc6-dbg:i386).
valgrind:  
valgrind:  Cannot continue -- exiting now.  Sorry.