Avoid some warnings during compilation (#8768)

Author: remkos

src/gmt_init.c

@@ -4986,7 +4986,7 @@ GMT_LOCAL int gmtinit_parse5_B_option (struct GMT_CTRL *GMT, char *in) {
 			GMT->current.setting.map_frame_type = GMT_IS_PLAIN;	/* A no-frame fancy would be super complicated */
 			GMT->current.setting.map_frame_pen.rgb[3] = 1.0;	/* Since it is very hard to no plot the axis, just make it transparent. */
 		}
-		else if (text[0] == '0' && !text[1] || !strncmp(text, "00", 2)) {	 /* Understand format '00' to mean zero line width frame. */
+		else if ((text[0] == '0' && !text[1]) || !strncmp(text, "00", 2)) {	 /* Understand format '00' to mean zero line width frame. */
 			GMT->current.map.frame.draw = true;			/* But we do wish to draw the frame */
 			if (GMT->common.J.zactive) GMT->current.map.frame.drawz = true;	/* Also brings z-axis into contention */
 			GMT->current.setting.map_frame_type = GMT_IS_PLAIN;	/* Since checkerboard without intervals look stupid */

src/grdinterpolate.c

@@ -500,7 +500,7 @@ EXTERN_MSC int GMT_grdinterpolate (void *V_API, int mode, void *args) {
 	}
 	else if (Ctrl->S.active) {	/* Create time/depth-series and not grid output */
 		/* Since we let grdtrack read the grids we do a separate branch here and the return from the module */
-		uint64_t seg, row;
+		uint64_t seg;
 		uint64_t dim[4] = {1, 1, 1, 2};	/* Dataset dimension for one point */
 		char header[GMT_LEN256] = {""};
 		struct GMT_DATASEGMENT *Si = NULL;

src/potential/gmtgravmag3d.c

@@ -511,8 +511,6 @@ GMT_LOCAL int read_xyz(struct GMT_CTRL *GMT, struct GMTGRAVMAG3D_CTRL *Ctrl, str
 	int n_cols = 0, error;
 	unsigned int k, n = 0;
 	size_t n_alloc = 10 * GMT_CHUNK;
-	char line[GMT_LEN256] = {""};
-	double x1, x2, x3;
 	struct GMT_RECORD *In = NULL;
 
 	if ((error = GMT_Set_Columns (GMT->parent, GMT_IN, 0, GMT_COL_VAR)) != GMT_NOERROR)