[BUG] Segfault unmetafying empty string

[dana <dana@dana.is>]

This crashes the shell for me on 5.4.2 and master:

  % compdef _foo foo
  % _foo() { local -a x; : <<< ${(F)x/y} }
  % foo <TAB>

(For some reason i can only replicate it during completion; idk why)

It seems like it's touching read-only memory trying to unmetafy an empty
string. Not sure where's best to deal with that

dana


#0  0x000055b20caa9460 in unmetafy (s=0x55b20cabddda "", len=0x7fff1e0485a0)
    at utils.c:4836
#1  0x000055b20ca23eb7 in getherestr (fn=0x7f124bbb2248) at exec.c:4514
#2  0x000055b20ca2145b in execcmd_exec (state=0x7fff1e048e30,
    eparams=0x7fff1e048a50, input=0, output=0, how=18, last1=2,
    close_if_forked=-1) at exec.c:3631
#3  0x000055b20ca1c7e6 in execpline2 (state=0x7fff1e048e30, pcode=67, how=18,
    input=0, output=0, last1=0) at exec.c:1927
#4  0x000055b20ca1b39c in execpline (state=0x7fff1e048e30, slcode=6146,
    how=18, last1=0) at exec.c:1658
#5  0x000055b20ca1a63f in execlist (state=0x7fff1e048e30, dont_change_job=1,
    exiting=0) at exec.c:1413
#6  0x000055b20ca19c7a in execode (p=0x55b20e865930, dont_change_job=1,
    exiting=0, context=0x55b20cab3042 "shfunc") at exec.c:1192
#7  0x000055b20ca282c5 in runshfunc (prog=0x55b20e865930, wrap=0x0,
    name=0x7f124bbb2168 "_foo") at exec.c:5974
#8  0x00007f124bfe4ebf in comp_wrapper (prog=0x55b20e865930, w=0x0,
    name=0x7f124bbb2168 "_foo") at complete.c:1524
#9  0x000055b20ca280a8 in runshfunc (prog=0x55b20e865930,
    wrap=0x7f124c203de0 <wrapper>, name=0x7f124bbb2168 "_foo") at exec.c:5958
#10 0x000055b20ca2788b in doshfunc (shfunc=0x55b20e8b7bf0,
    doshargs=0x7f124bbb6ba8, noreturnval=0) at exec.c:5824