x87: empty stack slot should not be infinity

Subject: https://github.com/rfalke/decompiler-subjects/blob/master/from_holdec/i386_x87_fpu/ia32_elf/subject.exe
Output:
```
int32_t FXAM(int32_t a1) {
int32_t v1 = __asm_fxam((float80_t)(float64_t)(int64_t)a1); // 0x804b347
__asm_wait();
printf("FXAM(para): should=0x0400 is=0x%04x\n", v1 & 0x4500);
int32_t v2 = __asm_fxam(3.141592653589793116L); // 0x804b368
__asm_wait();
printf("FXAM(pi): should=0x0400 is=0x%04x\n", v2 & 0x4500);
int32_t v3 = __asm_fxam(0.0L); // 0x804b389
__asm_wait();
printf("FXAM(0.0): should=0x4000 is=0x%04x\n", v3 & 0x4500);
int32_t v4 = __asm_fxam(NAN); // 0x804b3ae
__asm_wait();
printf("FXAM(nan): should=0x0100 is=0x%04x\n", v4 & 0x4500);
int32_t v5 = __asm_fxam(INFINITY); // 0x804b3d3
__asm_wait();
printf("FXAM(inf): should=0x0500 is=0x%04x\n", v5 & 0x4500);
int32_t v6 = __asm_fxam(INFINITY); // 0x804b3f2
__asm_wait();
printf("FXAM(empty): should=0x4100 is=0x%04x\n", v6 & 0x4500);
return 0;
}
```
There are no values in the stack for the last fxam instruction. Therefore the call `int32_t v6 = __asm_fxam(INFINITY);` has the wrong argument.

Start with the linked x87 subject executable and reproduce the generated FXAM output, focusing on the final empty-stack case. Compare the emitted argument and status classification for FXAM(empty) with the expected 0x4100 result; done when an empty stack slot is no longer emitted as INFINITY.

Written by the indexing model from the issue text.