fix push! into empty RLEVector
fix push! into empty RLEVector

Fixes `push!` into an empty `RLEVector.
Check for empty vector while accessing the `end` element.

fixes #2

JuliaPackageMirrors/RLEVectors.jl,JuliaPackageMirrors/RLEVectors.jl,JuliaPackageMirrors/RLEVectors.jl