Don't check if (!str) in xStrdup

This effectively reverts "Stricter strdup." 4674b4a732

If str is NULL upon the calling of strdup(), it should crash with SIGSEGV.
Just let it crash. Adding the "if (!str) fail();" code serves nothing but
bloat.

XAlloc.c

@@ -44,9 +44,6 @@ void* xRealloc(void* ptr, size_t size) {
 }
 
 char* xStrdup(const char* str) {
-   if (!str) {
-      fail();
-   }
    char* data = strdup(str);
    if (!data) {
       fail();