cmd/compile/internal/gc: add nil check for closure call on wasm

This commit adds an explicit nil check for closure calls on wasm,
so calling a nil func causes a proper panic instead of crashing on the
WebAssembly level.

Change-Id: I6246844f316677976cdd420618be5664444c25ae
Reviewed-on: https://go-review.googlesource.com/127759


Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>