plot: Fix the bug that -M+r modifier doesn't set the pen thickness

[seisman]

The plot's -M documenation says (https://docs.generic-mapping-tools.org/6.6/plot.html#m):

+r - Specify a pen to simply draw a line instead of a filled box in the legend, but replace the color information with that from the fill settings (i.e., only the pen width is used as specified, the color is not used).

Based on the documentation, the +r modifier should control the pen thickness of legend entries, but it doesn't work as expected. In the following script, +r5p set the pen thickness to 5p, but 5p is silently ignored.

#!/usr/bin/env bash
# Demonstrate filling of area between two sinusoidal curves with a section of NaNs
gmt begin GMT_fill_curves
	gmt math -T0/720/5 -N3/0 T -C1 COSD -C2 3 MUL SIND 4 DIV -C1,2 1 T 200 250 INRANGE SUB 0 NAN MUL = both_NaN.txt
	gmt plot -Mc+gblue+r5p+l"Short @~l@~ exceeds long" -Bxa90g90+u@. -Byafg -BWStr -JX15c/3c -R0/720/-1.25/2.5 both_NaN.txt -Gred -l"Long @~l@~ exceeds short"
gmt end show

This PR fixes the bug.

[Esteban82]

It works for me.