This commit fixes two bizarrely related bugs:

1. The signatures for the call* functions were wrong, indicating that
they had only two pointer arguments instead of three. We didn't notice
because the call* functions are defined by a macro expansion, which go
vet doesn't see.

2. deferArgs on a defer object with a zero-sized frame returned a
pointer just past the end of the allocated object, which is illegal in
Go (and can cause the "sweep increased allocation count" crashes).

In a fascinating twist, these two bugs canceled each other out, which
is why I'm fixing them together. The pointer returned by deferArgs is
used in only two ways: as an argument to memmove and as an argument to
reflectcall. memmove is NOSPLIT, so the argument was unobservable.
reflectcall immediately tail calls one of the call* functions, which
are not NOSPLIT, but the deferArgs pointer just happened to be the
third argument that was accidentally marked as a scalar. Hence, when
the garbage collector scanned the stack, it didn't see the bad
pointer as a pointer.

I believe this was all ultimately benign. In principle, stack growth
during the reflectcall could fail to update the args pointer, but it
never points to the stack, so it never needs to be updated. Also in
principle, the garbage collector could fail to mark the args object
because of the incorrect call* signatures, but in all calls to
reflectcall (including the ones spelled "call" in the reflect package)
the args object is kept live by the calling stack.

Change-Id: Ic932c79d5f4382be23118fdd9dba9688e9169e28
Reviewed-on: https://go-review.googlesource.com/31654
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>