Catch if infinity returned by parse_jsconstant is nothing

[IanButterworth]

Fixes

  │││││││││┌ parse(str::String) @ JSON.Parser /Users/ian/Documents/GitHub/JSON.jl/src/Parser.jl:443
  ││││││││││┌ parse(str::String; dicttype::Type{Dict{String, Any}}, inttype::Type{Int64}, allownan::Bool, null::Nothing) @ JSON.Parser /Users/ian/Documents/GitHub/JSON.jl/src/Parser.jl:450
  │││││││││││┌ parse_value(pc::JSON.Parser.ParserContext{Dict{String, Any}, Int64, true, nothing}, ps::JSON.Parser.MemoryParserState) @ JSON.Parser /Users/ian/Documents/GitHub/JSON.jl/src/Parser.jl:164
  ││││││││││││┌ parse_number(pc::JSON.Parser.ParserContext{Dict{String, Any}, Int64, true, nothing}, ps::JSON.Parser.MemoryParserState) @ JSON.Parser /Users/ian/Documents/GitHub/JSON.jl/src/specialized.jl:148
  │││││││││││││ no matching method found `-(::Nothing)` (1/3 union split): JSON.Parser.:-(infinity::Union{Nothing, Bool, Float64})
  ││││││││││││└────────────────────
  ││││││││││││┌ parse_number(pc::JSON.Parser.ParserContext{Dict{String, Any}, Int64, true, nothing}, ps::JSON.Parser.MemoryParserState) @ JSON.Parser /Users/ian/Documents/GitHub/JSON.jl/src/specialized.jl:148
  │││││││││││││ no matching method found `-(::Nothing)` (1/3 union split): JSON.Parser.:-(infinity::Union{Nothing, Bool, Float64})::Union{Float64, Int64}
  ││││││││││││└────────────────────

[quinnj]

Should be "fixed" (obsolete) after #374