flash/nor/atsamv: fix clang static analyzer warning

Warning:	line 679, column 4
4th function call argument is an uninitialized value

Change-Id: If62d96e1595be945c8e17885bb402e820fb1ec7b
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5903
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Jonathan McDowell <noodles-openocd@earth.li>

src/flash/nor/atsamv.c

@@ -676,6 +676,9 @@ showall:
 		}
 		if ((who >= 0) && (((unsigned)who) < SAMV_NUM_GPNVM_BITS)) {
 			r = samv_get_gpnvm(target, who, &v);
+			if (r != ERROR_OK)
+				return r;
+
 			command_print(CMD, "samv-gpnvm%u: %u", who, v);
 			return r;
 		} else {