packet-epl.c: Fixup missing initializer warnings

packet-epl.c: Fixup missing initializer warnings

macOS Buildbot doesn't like {0} (probably because GNU C already provides {}
exactly for the purpose of initializing all members to zero/NULL/0.0.. etc)

Affected local type definitions now have a static intializer macro that uses
the correct amount of zeroes and braces (similar to PTHREAD_MUTEX_INITIALIZER)
Global type definitions have a memset to zero (Which isn't strictly correct,
but as the platforms we support all have all-bits-zero-nulls and IEEE 754
floats, it should be good enough. A separate change will attempt to disable
-Wmissing-field-initializers -Wmissing-braces globally and hopefully make
these workarounds unnecessary.

Change-Id: I30b0f679bbb8adb2dd7269c9f3bc19732e48212b
Reviewed-on: https://code.wireshark.org/review/21887
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Guy Harris <guy@alum.mit.edu>