MDBF-534: Coverity scan: fix client folder

```------------------------------
File: `mysqltest`
```

------------------------------
- Coverity (SIZEOF_MISMATCH):
  - https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728385&defectInstanceId=53074863&mergedDefectId=972322
    Function `qsort` have to use size of element that is `uchar *`

- Coverity (REVERSE_INULL):
  - https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728385&defectInstanceId=53074524&mergedDefectId=1519693&fileStart=3376&fileEnd=3625
    First check if null and then use `strlen`, not reversed.

- FALSE POSITIVES
  - Coverity (TAINTED_SCALAR):
    https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728385&defectInstanceId=53074760&mergedDefectId=1519321

  - Coverity (CHECKED_RETURN):
    - https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728385&defectInstanceId=53074692&mergedDefectId=971714
    - https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728385&defectInstanceId=53072839&mergedDefectId=971715

  - Coverity (FORWARD_NULL):
    There is already issued DBUG_ASSERT(query_end) few lines before
    https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728385&defectInstanceId=53074002&mergedDefectId=971916&eventId=53074002-5

  - Coverity (OVERRUN):
    - https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728385&defectInstanceId=53074470&mergedDefectId=1519697
    - https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728385&defectInstanceId=53074862&mergedDefectId=1520391
      `uint64_max` and `SIZE_MAX` (max for `size_t`) are same as `count` argument
      for `memcmp`.

  - Coverity (RESOURCE_LEAK):
    - https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728385&defectInstanceId=53074163&mergedDefectId=1519889&eventId=53074163-446

- INTENTION:
  - Coverity (SIZEOF_MISMATCH):
    - https://scan5.scan.coverity.com/reports.htm#v58936/p10357/fileInstanceId=231728385&defectInstanceId=53074650&mergedDefectId=1520109
      `len` argument is used only in printing so it is not making impact (may be removed as an alternative).
      In this example size of pointer (8B) is used, that is not the size of value that pointer points to.

client/mysqltest.cc

@@ -3563,9 +3563,11 @@ void do_system(struct st_command *command)
 /* returns TRUE if path is inside a sandbox */
 bool is_sub_path(const char *path, size_t plen, const char *sandbox)
 {
-  size_t len= strlen(sandbox);
-  if (!sandbox || !len || plen <= len || memcmp(path, sandbox, len - 1)
-      || path[len] != '/')
+  size_t len;
+  if (!sandbox)
+    return false;
+  len= strlen(sandbox);
+  if (plen <= len || memcmp(path, sandbox, len-1) || path[len] != '/')
     return false;
   return true;
 }
@@ -11696,7 +11698,7 @@ void dynstr_append_sorted(DYNAMIC_STRING* ds, DYNAMIC_STRING *ds_input,
 
   /* Sort array */
   qsort(lines.buffer, lines.elements,
-        sizeof(char**), (qsort_cmp)comp_lines);
+        sizeof(uchar *), (qsort_cmp)comp_lines);
 
   /* Create new result */
   for (i= 0; i < lines.elements ; i++)