#2336 dashtype " " crashes wxt terminal

[Karl Ratzsch]

There seems to be some sanitising missing with dashtype specifier strings.
The command

plot x dt " "

crashes gnuplot with the wxt terminal, and the windows terminal issues a series of warnings

 Warning: Direct2D back-end error 80070057; last command: 31

(the last number varies). qt is unaffected and gives plot with a solid line, same with png, pdfcairo gives a plot with no visible line.