lx_loader: treat the OS/2 null selector as a null pointer

gspac/gspcg pass 0003:0000 (selector index 0, RPL 3) as a null far
pointer; lxConvert1616to32 only special-cased addr1616 == 0 and asserted
on ldt[0]. Segment index 0 is the null selector by definition, so return
NULL. This unblocks the TMS340 C front end (gspac), optimizer (gspopt)
and code generator (gspcg) under 2ine.

lx_loader.c

@@ -542,6 +542,8 @@ static void *lxConvert1616to32(const uint32 addr1616)
 
     const uint16 selector = (uint16) (addr1616 >> 19);  // slide segment down, and shift out control bits.
     const uint16 offset = (uint16) (addr1616 % 0x10000);  // all our LDT segments start at 64k boundaries (at the moment!).
+    if (selector == 0)  // segment 0x0000 or 0x0003: the OS/2 null selector.
+        return NULL;
     //printf("lxConvert1616to32: 0x%X -> %p\n", (uint) addr1616, (void *) (size_t) (GLoaderState.ldt[selector] + offset));
     assert(GLoaderState.ldt[selector] != 0);
     return (void *) (size_t) (GLoaderState.ldt[selector] + offset);