fixes a hanging swaygrab

https://github.com/SirCmpwn/sway/issues/1350 is fixed with this.
The commit that change swaygrab to use fork instead of Popen tried to
write to the read end of the pipe in the child branch and exec in the
parent branch.
This commit fixes both of those and closes the write fd after writing,
so convert actually exits.

swaygrab/main.c

@@ -58,14 +58,19 @@ void grab_and_apply_magick(const char *file, const char *payload,
 	if ((child = fork()) < 0) {
 		sway_log(L_ERROR, "Swaygrab failed to fork.");
 		exit(EXIT_FAILURE);
-	} else if (child == 0) {
+	} else if (child != 0) {
+		close(fd[0]);
+		write(fd[1], pixels, len);
 		close(fd[1]);
-		write(fd[0], pixels, len);
 		free(pixels - 9);
 		waitpid(child, NULL, 0);
 	} else {
+		close(fd[1]);
+		if (dup2(fd[0], 0) != 0) {
+			sway_log(L_ERROR, "Could not fdup the pipe");
+		}
 		close(fd[0]);
-		execlp("convert", "-depth", "8", "-size", size, "rgba:-", "-flip", file, NULL);
+		execlp("convert", "convert", "-depth", "8", "-size", size, "rgba:-", "-flip", file, NULL);
 		sway_log(L_ERROR, "Swaygrab could not run convert.");
 		exit(EXIT_FAILURE);
 	}