Fix comparisons of pointers with zero to compare with NULL instead.

Per C standard, these are semantically the same thing; but saying NULL
when you mean NULL is good for readability.

Marti Raudsepp, per results of INRIA's Coccinelle.

src/port/dirmod.c

@@ -246,7 +246,7 @@ pgsymlink(const char *oldpath, const char *newpath)
 	else
 		strcpy(nativeTarget, oldpath);
 
-	while ((p = strchr(p, '/')) != 0)
+	while ((p = strchr(p, '/')) != NULL)
 		*p++ = '\\';
 
 	len = strlen(nativeTarget) * sizeof(WCHAR);