Bug#11764310 - 57132: CONV FUNCTION CRASHES, NEGATIVE ARGUMENT TO MEMCPY

Amendment to previous patch:
Failure in CONV() should return NULL instead of
empty set.
When compiled on Windows or Solaris the function
Item_func_conv::val_str() doesn't fail on 
longlong2str() but finds an earlier exit path
based on the attributes of the arguments.
This exit path returns NULL on failure and as a
consequence the original patch caused different
test results depending on the OS used.

mysql-test/r/func_str.result

@@ -2789,7 +2789,7 @@ DROP TABLE t1;
 #
 SELECT CONV(1,-2147483648,-2147483648);
 CONV(1,-2147483648,-2147483648)
-
+NULL
 #
 # End of 5.5 tests
 #

sql/item_strfunc.cc

@@ -2954,7 +2954,10 @@ String *Item_func_conv::val_str(String *str)
 
   if (!(ptr= longlong2str(dec, ans, to_base)) ||
       str->copy(ans, (uint32) (ptr - ans), default_charset()))
-    return make_empty_result();
+  {
+    null_value= 1;
+    return NULL;
+  }
   return str;
 }