Fix for MDEV-21920

when converting (or copying) from string empty string weren't handled
correctly. This was a regression error, introduced by a prior covscan fix.

libmariadb/ma_stmt_codec.c

@@ -579,7 +579,7 @@ static void convert_froma_string(MYSQL_BIND *r_param, char *buffer, size_t len)
     case MYSQL_TYPE_NEWDECIMAL:
     default:
     {
-      if (len > r_param->offset)
+      if (len >= r_param->offset)
       {
         char *start= buffer + r_param->offset; /* stmt_fetch_column sets offset */
         char *end= buffer + len;

unittest/libmariadb/ps_bugs.c

@@ -5161,7 +5161,42 @@ static int test_maxparam(MYSQL *mysql)
   return OK;
 }
 
+static int test_mdev_21920(MYSQL *mysql)
+{
+  MYSQL_STMT *stmt= mysql_stmt_init(mysql);
+  MYSQL_BIND bind[1];
+  int rc;
+  char buffer[128];
+
+  rc= mysql_stmt_prepare(stmt, SL("SELECT ''"));
+  check_stmt_rc(rc, stmt);
+
+  rc= mysql_stmt_execute(stmt);
+  check_stmt_rc(rc, stmt);
+
+  buffer[0]= 1;
+
+  memset(bind, 0, sizeof(MYSQL_BIND));
+  bind[0].buffer_type= MYSQL_TYPE_STRING;
+  bind[0].buffer= buffer;
+  bind[0].buffer_length= 127;
+
+  rc= mysql_stmt_bind_result(stmt, bind);
+  check_stmt_rc(rc, stmt);
+
+  rc= mysql_stmt_fetch(stmt);
+  check_stmt_rc(rc, stmt);
+
+  FAIL_IF(buffer[0] != 0, "Expected empty string");
+
+
+  mysql_stmt_close(stmt);
+
+  return OK; 
+}
+
 struct my_tests_st my_tests[] = {
+  {"test_mdev_21920", test_mdev_21920, TEST_CONNECTION_DEFAULT, 0, NULL, NULL},
   {"test_maxparam", test_maxparam, TEST_CONNECTION_NEW, 0, NULL, NULL},
   {"test_conc424", test_conc424, TEST_CONNECTION_NEW, 0, NULL, NULL},
   {"test_conc344", test_conc344, TEST_CONNECTION_NEW, 0, NULL, NULL},