Stackoverflow if manifest contains references to removed drive

[SimonDanisch]

I moved a project to another drive, which I then removed, but the Manifest still contained one absolute path pointing to the old drive, which lead to:

pkg> add MathTeXEngine
    Updating registry at `C:\Users\sdani\.julia\registries\General`
    Updating git-repo `https://github.com/JuliaRegistries/General.git`
   Resolving package versions...
ERROR: StackOverflowError:

The reason is, that safe_realpath recursion anchor is ispath, which returns false for paths from a removed drive.

[barucden]

I think the following has the same underlying issue:

julia> safe_realpath("")
ERROR: StackOverflowError:
Stacktrace:
  [1] exec(re::Ptr{Nothing}, subject::String, offset::Int64, options::UInt32, match_data::Ptr{Nothing})
    @ Base.PCRE ./pcre.jl:193
  [2] exec_r_data
    @ ./pcre.jl:210 [inlined]
  [3] match(re::Regex, str::String, idx::Int64, add_opts::UInt32)
    @ Base ./regex.jl:371
  [4] match
    @ ./regex.jl:369 [inlined]
  [5] match
    @ ./regex.jl:388 [inlined]
  [6] _splitdir_nodrive(a::String, b::String)
    @ Base.Filesystem ./path.jl:138
  [7] splitdir
    @ ./path.jl:132 [inlined]
  [8] safe_realpath(path::String)
    @ Main ~/julia/Pkg.jl/src/utils.jl:68
  [9] safe_realpath(path::String) (repeats 14503 times)
    @ Main ~/julia/Pkg.jl/src/utils.jl:69
 [10] top-level scope
    @ REPL[2]:1

When ispath(path) returns false for any "sub-path", which is the case with a disconnected drive, it eventually arrives at splitdir(path) == ("", "C:"). The empty string is then passed to safe_realpath, causing the stackoverflow. I will prepare a PR.