findnext should use iteration

[Keno]

Currently, it tries to access invalid elements:

julia> findfirst(x->x=='0', "⨳⨳")
ERROR: UnicodeError: invalid character index
 in next(::String, ::Int64) at ./unicode/utf8.jl:65
 in getindex at ./strings/basic.jl:58 [inlined]
 in findnext(::##25#26, ::String, ::Int64) at ./array.jl:748
 in findfirst(::Function, ::String) at ./array.jl:754
 in eval(::Module, ::Any) at ./boot.jl:225
 in macro expansion at ./REPL.jl:92 [inlined]
 in (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at ./event.jl:46

[vtjnash]

#15755