popen: handle issues with fd0/1 being closed

also check for failure of dup2 and abort the child rather than
reading/writing the wrong file.

src/stdio/popen.c

@@ -31,9 +31,9 @@ FILE *popen(const char *cmd, const char *mode)
 		close(p[1]);
 		return NULL;
 	case 0:
-		dup2(p[1-op], 1-op);
-		close(p[0]);
-		close(p[1]);
+		if (dup2(p[1-op], 1-op) < 0) _exit(127);
+		if (p[0] != 1-op) close(p[0]);
+		if (p[1] != 1-op) close(p[1]);
 		execl("/bin/sh", "sh", "-c", cmd, (char *)0);
 		_exit(127);
 	}