Fix invalidation with pointer(::String, ::Integer) (#37820)

Author: omus

base/strings/string.jl

@@ -92,7 +92,7 @@ String(s::CodeUnits{UInt8,String}) = s.s
 ## low-level functions ##
 
 pointer(s::String) = unsafe_convert(Ptr{UInt8}, s)
-pointer(s::String, i::Integer) = pointer(s)+(i-1)
+pointer(s::String, i::Integer) = pointer(s) + Int(i)::Int - 1
 
 @pure ncodeunits(s::String) = Core.sizeof(s)
 codeunit(s::String) = UInt8