MDBF-534: Coverity scan: fix client folder

```------------------------------
File: `mysql`
```

------------------------------

- Coverity (RESOURCE_LEAK):
  https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728394&defectInstanceId=53073025&mergedDefectId=1520090&eventId=53073025-15

  `mysql`: memory allocated by `mysql_fetch_row` is not freed.

- FALSE POSITIVES:
  - Coverity (TAINTED_SCALAR):
    - https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728394&defectInstanceId=53074559&mergedDefectId=1520403
  - Coverity (COPY_PASTE_ERROR):
    https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728394&defectInstanceId=53074521&mergedDefectId=1520300
  - Coverity (STRING_NULL):
    https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728394&defectInstanceId=53072524&mergedDefectId=1519374
  - Coverity (CHECKED_RETURN):
    https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728394&defectInstanceId=53074932&mergedDefectId=971708

- INTENTIONAL:
  - Coverity (UNINIT):
    https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728394&defectInstanceId=53074758&mergedDefectId=1519932
    https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728394&defectInstanceId=53073939&mergedDefectId=1519738
  - Coverity(BAD_FREE):
    https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728394&defectInstanceId=53073938&mergedDefectId=1519491
    https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728394&defectInstanceId=53074819&mergedDefectId=1519462

---------------------------------
File: `mysql_plugin`
---------------------------------

- Coverity (FORWARD_NULL):
  https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728420&defectInstanceId=53074485&mergedDefectId=971915

  Dereference after null check when using `fclose`.

- FALSE POSITIVES:
  - Coverity (STRING_OVERFLOW):
    https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728420&defectInstanceId=53075014&mergedDefectId=972410

- Additionally fix typo

client/mysql.cc

@@ -5357,6 +5357,7 @@ static void init_username()
     full_username=my_strdup(cur[0],MYF(MY_WME));
     part_username=my_strdup(strtok(cur[0],"@"),MYF(MY_WME));
     (void) mysql_fetch_row(result);		// Read eof
+    mysql_free_result(result);
   }
 }
 

sql/sql_string.cc

@@ -553,7 +553,7 @@ bool String::append(const char *s,size_t size)
   }
 
   /*
-    For an ASCII compatinble string we can just append.
+    For an ASCII compatible string we can just append.
   */
   return Binary_string::append(s, arg_length);
 }