Correct C FixedMul() off-by-one errors

return (fixed_t)(((UINT64)((INT64)a * b)) >> FRACBITS);

#include <stdint.h>
#include <stdio.h>

#define FRACBITS 16
#define FRACUNIT (1<<FRACBITS)

typedef int32_t INT32;
typedef int64_t INT64;
typedef INT32 fixed_t;

fixed_t FixedMul(fixed_t a, fixed_t b)
{
	return (fixed_t)((((INT64)a * b) ) / FRACUNIT);
}

fixed_t FixedMulWorking(fixed_t a, fixed_t b)
{
	return (fixed_t)((((INT64)a * b) ) >> FRACBITS);
}

fixed_t FixedMulAsm(fixed_t a, fixed_t b) // asm
{
    fixed_t ret;
    asm
	(
		 "imull %2;"           // a*b
		 "shrdl %3,%%edx,%0;"  // shift logical right FRACBITS bits
		:"=a" (ret)            // eax is always the result and the first operand (%0,%1)
		:"0" (a), "r" (b)      // and %2 is what we use imull on with what in %1
		, "I" (FRACBITS)       // %3 holds FRACBITS (normally 16)
		:"%cc", "%edx"         // edx and condition codes clobbered
	);
	return ret;
}

int main()
{
    fixed_t a = 16384;
    fixed_t b = -49261;
    printf("FixedMul: %i\n", FixedMul(a, b));
    printf("FixedMulWorking: %i\n", FixedMulWorking(a, b));
    printf("FixedMulAsm: %i\n", FixedMulAsm(a, b));
    return 0;
}

FixedMul: -12315
FixedMulWorking: -12316
FixedMulAsm: -12316