MDEV-30351 crash in Item_func_left::val_str

When using LEFT() function with a string that is without a charset,
the function crashes. This is because the function assumes that
the string has a charset, and tries to use it to calculate the
length of the string.

Two functions, UNHEX and WEIGHT_STRING, returned a string without
the charset being set to a not null value.

The fix is to set charset when calling val_str on these two functions.
Reviewed-by: Alexander Barkov <bar@mariadb.com>
Reviewed-by: Daniel Black <daniel@mariadb.org>

mysql-test/main/func_str.result

@@ -5240,6 +5240,15 @@ DROP TABLE crash_test_2;
 # Start of 10.4 tests
 #
 #
+# MDEV-30351 crash in Item_func_left::val_str
+#
+SELECT WEIGHT_STRING('aa') IN (LEFT(WEIGHT_STRING('aaa'),4),'bbb') as expect_1;
+expect_1
+1
+SELECT UNHEX('0032') in (LEFT(UNHEX('003200'), 2),'dog') as expect_1;
+expect_1
+1
+#
 # MDEV-21841 CONV() function truncates the result type to 21 symbol.
 #
 CREATE TABLE t1(i BIGINT);

mysql-test/main/func_str.test

@@ -2256,16 +2256,22 @@ CREATE TABLE crash_test_2 (
 DROP TABLE t1;
 DROP TABLE crash_test_2;
 
-
 --echo #
 --echo # End of 10.3 tests
 --echo #
 
-
 --echo #
 --echo # Start of 10.4 tests
 --echo #
 
+--echo #
+--echo # MDEV-30351 crash in Item_func_left::val_str
+--echo #
+
+SELECT WEIGHT_STRING('aa') IN (LEFT(WEIGHT_STRING('aaa'),4),'bbb') as expect_1;
+
+SELECT UNHEX('0032') in (LEFT(UNHEX('003200'), 2),'dog') as expect_1;
+
 --echo #
 --echo # MDEV-21841 CONV() function truncates the result type to 21 symbol.
 --echo #

sql/item_strfunc.cc

@@ -3729,6 +3729,7 @@ String *Item_func_weight_string::val_str(String *str)
                                  flags);
   DBUG_ASSERT(frm_length <= tmp_length);
 
+  str->set_charset(&my_charset_bin);
   str->length(frm_length);
   null_value= 0;
   return str;
@@ -3808,6 +3809,7 @@ String *Item_func_unhex::val_str(String *str)
 
   from= res->ptr();
   null_value= 0;
+  str->set_charset(&my_charset_bin);
   str->length(length);
   to= (char*) str->ptr();
   if (res->length() % 2)