Minor debug fixes

PSprite.c: move the trace message up before the label, so if tb is NULL there isn't a NULL pointer dereference
guard.c: refactor assert() to not check ptr!=0 a second time
Found by cppcheck static analysis.
Tagged as Paint-2_23-1 since both are debug code only.

c/PSprite

@@ -833,8 +833,8 @@ main_ttab *psprite_ttab_new (main_sprite *sprite, int dmode, int *dpal)
     tb->ttab2_size = 0;
   }
 
-finish:
   ftracef2 ("table 0x%X, table2 0x%X\n", tb->table, tb->table2);
+finish:
 
   if (msg != NULL)
   { if (done_table2 && tb->ttab2_size != 0)

c/guard

@@ -249,7 +249,7 @@ void guard_free (char *file, int line, void *ptr, size_t size)
    {  header = (Block_Header *) ptr - 1;
       actual_size = header->size;
 
-      assert (!(ptr == 0 || (int) ptr < 0));
+      assert ((int) ptr > 0);
       assert (header->guard == GUARD_WORD); /*protects against multiple
             freeing*/
       assert (size == 0 || actual_size == size);