Fix null pointer subtraction warning (#10498)

The warning:
```
pqsort.c:106:7: warning: performing pointer subtraction with a null pointer has undefined behavior [-Wnull-pointer-subtraction]
loop:   SWAPINIT(a, es);
        ^~~~~~~~~~~~~~~
pqsort.c:65:47: note: expanded from macro 'SWAPINIT'
#define SWAPINIT(a, es) swaptype = ((char *)a - (char *)NULL) % sizeof(long) || \
```
Clang version:
```
Apple clang version 13.1.6 (clang-1316.0.21.2)
Target: x86_64-apple-darwin21.3.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
```

src/pqsort.c

@@ -38,7 +38,7 @@
  */
 
 #include <sys/types.h>
-
+#include <stdint.h>
 #include <errno.h>
 #include <stdlib.h>
 
@@ -62,7 +62,7 @@ static inline void	 swapfunc (char *, char *, size_t, int);
         } while (--i > 0);				\
 }
 
-#define SWAPINIT(a, es) swaptype = ((char *)a - (char *)0) % sizeof(long) || \
+#define SWAPINIT(a, es) swaptype = (uintptr_t)a % sizeof(long) || \
 	es % sizeof(long) ? 2 : es == sizeof(long)? 0 : 1;
 
 static inline void