Fix uninitialized warning, and work around a bug in gcc over clobbering

Summary:
r329077 caused gcc to emit uninitialized use warnings. Attempting to
fix those warnings yielded the following warnings:

usr.bin/tftp/main.c: In function 'main':
usr.bin/tftp/main.c:181: warning: variable 'el' might be clobbered by
'longjmp' or 'vfork'
usr.bin/tftp/main.c:182: warning: variable 'hist' might be clobbered by
'longjmp' or 'vfork'

This is a known bug in gcc, found at
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24239

Work around that by simply marking hist and el as static.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D14302