Fix bugs revealed by GCDEBUG.

[jpco]

Three classes of error with GCDEBUG (or just GCALWAYS), which probably represent latent memory unsafety:

• segfault in $&fsplit (which causes crash on startup)
• segfault in $&catch (which causes crash on exit)
• segfault in tilde handling (which causes either a crash or the inability to actually expand ~ to the output of %home)

It's unbelievable that I've taken 5-6 years to actually submit this pull request.

Fixes wryun/es-shell#18

[jpco]

Sorry, I am noisily being bad at Github.