fix potential null deref in merge_vararg_unions

[adienes]

so as to avoid segfaults in the following type normalization

julia> @test Union{Tuple{}, Tuple{Int}, Tuple{UInt}} <: Union{
        Tuple{},
        Tuple{Int},
        Tuple{UInt},
        Tuple{UInt, Vararg{UInt}},
        Tuple{Int, Int, Vararg{Int}}
    }

[89686] signal 11 (2): Segmentation fault: 11
in expression starting at REPL[2]:1
jl_is_tuple_type at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-XG3Q6T6R70.0/build/default-honeycrisp-XG3Q6T6R70-0/julialang/julia-master/src/./julia.h:1835 [inlined]
merge_vararg_unions at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-XG3Q6T6R70.0/build/default-honeycrisp-XG3Q6T6R70-0/julialang/julia-master/src/jltypes.c:617
ijl_type_union at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-XG3Q6T6R70.0/build/default-honeycrisp-XG3Q6T6R70-0/julialang/julia-master/src/jltypes.c:674
jl_apply at /Users/julia/.julia/scratchspaces/a66863

[Seelengrab]

Suggested change

	@test Union{Tuple{}, Tuple{Int}, Tuple{UInt}} <: Union{
	# This is a regression test against a nullcheck/segfault in jltypes.c
	@test Union{Tuple{}, Tuple{Int}, Tuple{UInt}} <: Union{

[adienes]

it's also a correct test, right? I mean I'm not opposed to such a comment per se it just seems maybe unnecessary, especially since the cause of the crash is "typo" not "design failure"

[vtjnash]

Preferably it would have been added to the list of tests for this function, with a comment noting the issue number or PR, though both are commonly extracted from git praise anyways d4362e4#diff-90c3a7a047872b9929e6577e16ad589a77552d0de0400c98cf303b6b7204e683R8257