prevent crash in kinc_g4_set_matrix4()

[sh-dave]

Due to the uninitialized variable, any of the kinc_g4_set_xxx() would crash later

Kinc/Backends/Graphics4/G4onG5/Sources/kinc/backend/graphics4/G4.c.h

Lines 443 to 450 in 83af1a7

	void kinc_g4_set_float(kinc_g4_constant_location_t location, float value) {
	if (location.impl._location.impl.vertexOffset >= 0)
	kinc_g5_constant_buffer_set_float(&vertexConstantBuffer, location.impl._location.impl.vertexOffset, value);
	if (location.impl._location.impl.fragmentOffset >= 0)
	kinc_g5_constant_buffer_set_float(&fragmentConstantBuffer, location.impl._location.impl.fragmentOffset, value);
	if (location.impl._location.impl.computeOffset >= 0)
	kinc_g5_constant_buffer_set_float(&computeConstantBuffer, location.impl._location.impl.computeOffset, value);
	}