No, it's not undefined behavior. Only the status returned to the host environment is undefined. (And in C99 and later, the status is 0.)

As for your example:

int foo(int x) { if (x >= 0) return 123; if (x < 0) return 456; }

that has undefined behavior only if the caller uses the value. (That's to cater to ancient C code written before the void type was introduced.)