This circuit should give you the behavior you're looking for (updated):

^{simulate this circuit – Schematic created using CircuitLab}

In the default state with no buttons pressed, both sides of the motor are grounded and nothing happens.
If you push the SW1 Up or SW2 Up, then the left relay activates and connects +12V to the left side of the motor, and it runs in one direction.
If you push SW1 Down or SW2 Down, then the right relay activates and connects 12V to the right side of the motor, and it runs in the other direction.
If you press both an Up and a Down button together, then both relays activate and both sides of the motor are connected to +12V - and nothing happens.

So there's no set of button-presses which will result in a blown fuse.

This circuit should give you the behavior you're looking for (updated):

^{simulate this circuit – Schematic created using CircuitLab}

In the default state with no buttons pressed, both sides of the motor are grounded and nothing happens.
If you push the SW1 Up or SW2 Up, then the left relay activates and connects +12V to the left side of the motor, and it runs in one direction.
If you push SW1 Down or SW2 Down, then the right relay activates and connects 12V to the right side of the motor, and it runs in the other direction.
If you press both an Up and a Down button together, then both relays activate and both sides of the motor are connected to +12V - and nothing happens.

So there's no set of button-presses which will result in a blown fuse.

This circuit should give you the behavior you're looking for:

^{simulate this circuit – Schematic created using CircuitLab}

In the default state with no buttons pressed, both sides of the motor are grounded and nothing happens.
If you push the left button, then the left relay activates and connects +12V to the left side of the motor, and it runs in one direction.
If you push the right button, then the right relay activates and connects 12V to the right side of the motor, and it runs in the other direction.
If you press both buttons, then both relays activate and both sides of the motor are connected to +12V - and nothing happens.

This circuit should give you the behavior you're looking for (updated):

^{simulate this circuit – Schematic created using CircuitLab}

In the default state with no buttons pressed, both sides of the motor are grounded and nothing happens.
If you push the SW1 Up or SW2 Up, then the left relay activates and connects +12V to the left side of the motor, and it runs in one direction.
If you push SW1 Down or SW2 Down, then the right relay activates and connects 12V to the right side of the motor, and it runs in the other direction.
If you press both an Up and a Down button together, then both relays activate and both sides of the motor are connected to +12V - and nothing happens.

So there's no set of button-presses which will result in a blown fuse.