pipeto: check argv[0] is set

This path couldn't so far be triggered, but can be useful if we reuse
this code.

pipeto.c

@@ -51,7 +51,10 @@ pipeto(const char *cmdline)
 		}
 		argv[argc] = 0;
 
-		execvp(argv[0], argv);
+		if (argv[0])
+			execvp(argv[0], argv);
+		else
+			errno = EINVAL;
 
 		// execvp failed, write errno to parent
 		long e = errno;