Fix bug of to_ascii constructor from vectors and pointers

[jessymilare]

Correct a few typos that prevents creation of ascii strings from vectors and pointers. E.g.:

julia> v = collect(codeunits("asdfghjkl\0"))
10-element Array{UInt8,1}:
 0x61
 0x73
 0x64
 0x66
 0x67
 0x68
 0x6a
 0x6b
 0x6c
 0x00

julia> to_ascii(v)
ERROR: UndefVarError: str not defined
Stacktrace:
 [1] isascii(::Array{UInt8,1}) at C:\Users\jessy\.julia\packages\StrBase\3wQxZ\src\utf8.jl:158
 [2] convert(::Type{ASCIIStr}, ::Array{UInt8,1}) at C:\Users\jessy\.julia\packages\StrBase\3wQxZ\src\ascii.jl:52
 [3] to_ascii(::Array{UInt8,1}) at C:\Users\jessy\.julia\packages\StrBase\3wQxZ\src\legacy.jl:6
 [4] top-level scope at REPL[109]:1

julia> ASCIIStr(v)
ERROR: UndefVarError: str not defined
Stacktrace:
 [1] isascii(::Array{UInt8,1}) at C:\Users\jessy\.julia\packages\StrBase\3wQxZ\src\utf8.jl:158
 [2] convert(::Type{ASCIIStr}, ::Array{UInt8,1}) at C:\Users\jessy\.julia\packages\StrBase\3wQxZ\src\ascii.jl:52
 [3] ASCIIStr(::Array{UInt8,1}) at C:\Users\jessy\.julia\packages\StrBase\3wQxZ\src\types.jl:55
 [4] top-level scope at REPL[110]:1

julia> to_ascii(pointer(v))
ERROR: UndefVarError: p not defined
Stacktrace:
 [1] to_ascii(::Ptr{UInt8}) at C:\Users\jessy\.julia\packages\StrBase\3wQxZ\src\legacy.jl:7
 [2] top-level scope at REPL[111]:1

[jessymilare]

I believe the error is unrelated to the changes I've made.

[ScottPJones]

Thanks for this! I should have tested the "legacy" constructors more!