Fixed alias bug when compiling with gcc 4.2.4 that caused subselect.test to fail

sql/item.cc:
  Removed alias warnings by changing type from char * to const char*
sql/item.h:
  Removed alias warnings by changing type from char * to const char*
sql/item_subselect.cc:
  Fixed alias bug when compiling with gcc 4.2.4 that caused subselect.test to fail
sql/sql_string.h:
  Removed alias warnings by changing type from char * to const char*
storage/heap/hp_test2.c:
  Removed SAFEMALLOC to get rid of compiler error
  Fixed test case as we can't anymore use heap_rlast() on a HASH key entry.