Fix concatenation of string and char in splitdir

This fixes:
julia> dirname("/tmp")
ERROR: no method *(ASCIIString,Char)
 in splitdir at path.jl:32
 in dirname at path.jl:37

We may want to consider getting rid of the remaining * concatenations
in path.jl, but in brief tests I was not able to trigger errors. So
this is conservative.