cppcheck warns that vx >> 31 is undefined behaviour when vx is signed.

More generally, bitwise operations on signed integers are implementation
defined behaviour.
See https://wiki.sei.cmu.edu/confluence/display/c/INT13-C.+Use+bitwise+operators+only+on+unsigned+operands

So use the ( quint32 )( -( qint32 )( ( quint32 )vx >> 31 ) ) ) trick instead.

Demo:
- if vx is negative, (quint32)vx >> 31 leads to 1, which then becomes -1,
  which then becomes 0xFFFFFFFF when cast to unsigned.
- if vx is >= 0, this leads to 0.

gcc compiles both non-compliant and compliant versions identically. Given
test.cpp with
```
unsigned foo(int x)
{
    return x >> 31;
}

unsigned bar(int x)
{
    return (unsigned)(-(int)((unsigned)x >> 31));
}
```

g++ -O2 test.cpp -c && objdump -d test.o :

```
0000000000000000 <_Z3fooi>:
   0:	89 f8                	mov    %edi,%eax
   2:	c1 f8 1f             	sar    $0x1f,%eax
   5:	c3                   	retq
   6:	66 2e 0f 1f 84 00 00 	nopw   %cs:0x0(%rax,%rax,1)
   d:	00 00 00

0000000000000010 <_Z3bari>:
  10:	89 f8                	mov    %edi,%eax
  12:	c1 f8 1f             	sar    $0x1f,%eax
  15:	c3                   	retq
```