Fixed memory leak when doing :tcl ::vim::expr 2

[dpelle]

This PR fixes a memory leak in vim-8.1.146 and older.
The leak happens as meany times as the :tcl::vim::expr command is invoked.
For example, this will leak 1 block:

$ valgrind --leak-check=full 2> vg.log ./vim --clean -c'tcl ::vim::expr 2+3' -cq

And vg.log contains:

==19619== 2 bytes in 1 blocks are definitely lost in loss record 2 of 169
==19619==    at 0x4C2FDCB: malloc (vg_replace_malloc.c:299)
==19619==    by 0x1FC4C0: lalloc (misc2.c:976)
==19619==    by 0x1FD32D: alloc (misc2.c:874)
==19619==    by 0x1FD32D: vim_strsave (misc2.c:1315)
==19619==    by 0x16273E: eval_to_string (eval.c:844)
==19619==    by 0x2EC8C1: tclvimexpr (if_tcl.c:1384)
==19619==    by 0x663F405: TclNRRunCallbacks (in /usr/lib/x86_64-linux-gnu/libtcl8.6.so)
==19619==    by 0x663FFB9: ??? (in /usr/lib/x86_64-linux-gnu/libtcl8.6.so)
==19619==    by 0x663FA62: Tcl_EvalEx (in /usr/lib/x86_64-linux-gnu/libtcl8.6.so)
==19619==    by 0x6640C64: Tcl_Eval (in /usr/lib/x86_64-linux-gnu/libtcl8.6.so)
==19619==    by 0x2EE97E: ex_tcl (if_tcl.c:1922)
==19619==    by 0x19D191: do_one_cmd (ex_docmd.c:2886)
==19619==    by 0x19D191: do_cmdline (ex_docmd.c:1040)
==19619==    by 0x30416F: exe_commands (main.c:2939)
==19619==    by 0x30416F: vim_main2 (main.c:812)

The leak was found after adding more Tcl tests
in #3140

[codecov-io]

Codecov Report

Merging #3150 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3150      +/-   ##
==========================================
- Coverage   76.04%   76.04%   -0.01%     
==========================================
  Files          93       93              
  Lines      136733   136734       +1     
==========================================
- Hits       103983   103981       -2     
- Misses      32750    32753       +3

Impacted Files	Coverage Δ
src/if_tcl.c	27.06% <0%> (-0.04%)	⬇️
src/gui_gtk_x11.c	48.18% <0%> (-0.15%)	⬇️
src/os_unix.c	57.51% <0%> (-0.1%)	⬇️
src/gui.c	49.48% <0%> (-0.06%)	⬇️
src/message.c	74.83% <0%> (-0.05%)	⬇️
src/ex_docmd.c	77.54% <0%> (-0.02%)	⬇️
src/ex_cmds2.c	82.73% <0%> (+0.09%)	⬆️
src/if_xcmdsrv.c	84.71% <0%> (+0.71%)	⬆️

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