MDEV-15762 - VARCHAR(0) COMPRESSED crashes the server

Character set safe truncation is done when storing non-empty string in
VARCHAR(0) COMPRESSED column, so that string becomes empty. The code
didn't expect empty string after truncation.

Fixed by moving empty string check after truncation.

mysql-test/main/column_compression.result

@@ -1373,3 +1373,16 @@ bar
 foo
 SET SESSION optimizer_switch=DEFAULT;
 DROP TABLE t1;
+#
+# MDEV-15762 - VARCHAR(0) COMPRESSED crashes the server
+#
+CREATE TABLE t1(a VARCHAR(0) COMPRESSED);
+INSERT INTO t1 VALUES('a');
+ERROR 22001: Data too long for column 'a' at row 1
+INSERT INTO t1 VALUES(' ');
+Warnings:
+Note	1265	Data truncated for column 'a' at row 1
+SELECT LENGTH(a) FROM t1;
+LENGTH(a)
+0
+DROP TABLE t1;

mysql-test/main/column_compression.test

@@ -91,3 +91,14 @@ SET SESSION optimizer_switch = 'derived_merge=off';
 SELECT * FROM ( SELECT * FROM t1 ) AS sq ORDER BY b;
 SET SESSION optimizer_switch=DEFAULT;
 DROP TABLE t1;
+
+
+--echo #
+--echo # MDEV-15762 - VARCHAR(0) COMPRESSED crashes the server
+--echo #
+CREATE TABLE t1(a VARCHAR(0) COMPRESSED);
+--error ER_DATA_TOO_LONG
+INSERT INTO t1 VALUES('a');
+INSERT INTO t1 VALUES(' ');
+SELECT LENGTH(a) FROM t1;
+DROP TABLE t1;

sql/field.cc

@@ -7958,12 +7958,6 @@ int Field_longstr::compress(char *to, uint *to_length,
   char *buf= 0;
   int rc= 0;
 
-  if (length == 0)
-  {
-    *to_length= 0;
-    return 0;
-  }
-
   if (String::needs_conversion_on_storage(length, cs, field_charset) ||
       *to_length <= length)
   {
@@ -7981,10 +7975,11 @@ int Field_longstr::compress(char *to, uint *to_length,
                                     (*to_length - 1) / field_charset->mbmaxlen);
     rc= check_conversion_status(&copier, end, cs, true);
     from= buf;
-    DBUG_ASSERT(length > 0);
   }
 
-  if (length >= thd->variables.column_compression_threshold &&
+  if (length == 0)
+    *to_length= 0;
+  else if (length >= thd->variables.column_compression_threshold &&
       (*to_length= compression_method()->compress(thd, to, from, length)))
     status_var_increment(thd->status_var.column_compressions);
   else