segfault on mysql_escape_string

When `mysql_escape_string` is called without creating any connections:

```
Program received signal SIGSEGV, Segmentation fault.
mysql_cset_escape_slashes (cset=0x0, newstr=0x7ffff7590d21 "\vY\367\377", escapestr=escapestr@entry=0x7ffff7590898 "1",
escapestr_len=escapestr_len@entry=1) at ./libmariadb/libmariadb/ma_charset.c:1040
1040 ./libmariadb/libmariadb/ma_charset.c: No such file or directory.
(gdb) bt 5
#0 mysql_cset_escape_slashes (cset=0x0, newstr=0x7ffff7590d21 "\vY\367\377", escapestr=escapestr@entry=0x7ffff7590898 "1",
escapestr_len=escapestr_len@entry=1) at ./libmariadb/libmariadb/ma_charset.c:1040
#1 0x00007ffff74d4a08 in mysql_escape_string (to=<optimized out>, from=from@entry=0x7ffff7590898 "1", length=length@entry=1)
at ./libmariadb/libmariadb/mariadb_lib.c:3428
```

In case of MySQL, default character set should be used, instead of segv.