grdview: Fix the bug when -N is set but level is not specified

[seisman]

According to the documentation (https://docs.generic-mapping-tools.org/dev/grdview#n), when -N is set but -R isn't, the default level should be the minimum value in the grid.

-N[level][+gfill]
Draws a plane at this z-level. If the optional color is provided via the +g modifier, and the projection is not oblique, the frontal facade between the plane and the data perimeter is colored. See -Wf for setting the pen used for the outline. If no level is set then we default to the minimum value in the reliefgrid. However, if -R was used to set zmin/zmax then we use that value if it is less than the grid minimum value.

However, the default level is set to 0 due to a bug. This can be verified by the following test:

gmt begin map
    gmt grdview @static_earth_relief.nc -Jz0.005 -Wf0.5p,blue,dashed -p225/30 -Baf -Bzaf -N
    gmt grdview @static_earth_relief.nc -Jz0.005 -Wf0.5p,blue,dashed -p225/30 -Baf -Bzaf -N -R-55/-47/-24/-10/0/1000 -Xw+1c
    gmt grdview @static_earth_relief.nc -Jz0.005 -Wf0.5p,blue,dashed -p225/30 -Baf -Bzaf -N190 -Xw+1c
gmt end show

Actual output	Expected output

When -R is not specified, GMT first determines the z-range based on the grid min/max values, then checks if Ctrl->N.level is set and updates zmin to Ctrl->N.level if zmin<level, as shown below:

gmt/src/grdview.c

Lines 990 to 996 in d52a1e5

	/* Set default z-range for plot to be that of the grid if not specified via -R */
	if (GMT->common.R.wesn[ZLO] == 0.0 && GMT->common.R.wesn[ZHI] == 0.0) {
	GMT->common.R.wesn[ZLO] = Topo->header->z_min;
	GMT->common.R.wesn[ZHI] = Topo->header->z_max;
	if (Ctrl->N.active && Ctrl->N.level < GMT->common.R.wesn[ZLO]) GMT->common.R.wesn[ZLO] = Ctrl->N.level;
	if (Ctrl->N.active && Ctrl->N.level > GMT->common.R.wesn[ZHI]) GMT->common.R.wesn[ZHI] = Ctrl->N.level;
	}

However, when -N is set without any level, Ctrl->N.implicit is true and Ctrl->N.level is not set until

gmt/src/grdview.c

Line 1320 in d52a1e5

Ctrl->N.level = Topo->header->z_min;

so, Ctrl->N.level still has the initial value (0.0), and then zmin is updated to 0.0 in this case.

This PR fixes the issue.