startup: do not accept stray characters after a "+" on the command line

After a "+" only c, r, C, and R are valid characters,
and only when immediately followed by / or ?.

This fixes https://savannah.gnu.org/bugs/?60667.

Bug existed since version 4.4, commit 2326bf6,
since searching at startup with +/ or +? was introduced.

src/nano.c

@@ -2403,7 +2403,7 @@ int main(int argc, char **argv)
 				optind++;
 			} else
 #endif
-			if (!parse_line_column(&argv[optind++][n], &givenline, &givencol))
+			if (!parse_line_column(&argv[optind++][1], &givenline, &givencol))
 				statusline(ALERT, _("Invalid line or column number"));
 		}