string: fix out of bound access in perm_chars causing segfault

[julthomas]

In struct data_String, the perm_chars member is declared as
an array indexed from 0 to 255:

char perm_chars[256];

Bytes > 127, for instance with UTF-8 data, cause a segfault.
I believe casting as (unsigned char) instead of (unsigned) should
ensure the perm_chars is accessed at index 0..255.

I came this segfault with the following string sample. Rsyslog crashes
in stringIsPermittedChar() when it processes char byte 0xe2.

echo 'd’ouverture' |od -t x1 -a
0000000  64  e2  80  99  6f  75  76  65  72  74  75  72  65  0a
          d   b nul  em   o   u   v   e   r   t   u   r   e  nl
0000016

Note line numbers in parser.c reported by gdb are wrong with master
because it is from a 2.0.6 version with other patches:

#0  0x00007f9a73034713 in stringIsPermittedChar (data=0x55bd3a1a42a0, c=-30 '\342') at parser.c:3236
#1  0x00007f9a73034c63 in ln_v2_parseString (npb=0x7f9a5292d670, offs=0x7f9a5292d600, pdata=0x55bd3a1a42a0, parsed=0x7f9a5292d5f8, value=0x7f9a5292d5f0) at parser.c:3363
#2  0x00007f9a73029279 in tryParser (npb=0x7f9a5292d670, dag=0x55bd3a1a11a0, offs=0x7f9a5292d600, pParsed=0x7f9a5292d5f8, value=0x7f9a5292d5f0, prs=0x55bd3a14ea00) at pdag.c:1454
#3  0x00007f9a730295db in ln_normalizeRec (npb=0x7f9a5292d670, dag=0x55bd3a1a11a0, offs=0, bPartialMatch=0, json=0x7f9a4c4dd260, endNode=0x7f9a5292d6a0) at pdag.c:1575
#4  0x00007f9a730299ac in ln_normalize (ctx=0x55bd3a14e680, str=0x7f9a4c4e19a0 "d’ouverture de session :#011#011{837B856E-32B3-78E2-3D77-4E95F8904E71}#015#012#015#012Informations sur le processus :#015#012#011ID du processus :#011#0110x0#015#012#011Nom du processus :#011#01"..., strLen=8021, json_p=0x7f9a5292d6d8) at pdag.c:1653
#5  0x00007f9a7302459e in doAction (pMsgData=0x7f9a5292d740, pWrkrData=0x7f9a4c4daad0) at mmnormalize.c:259
...

This should fix issue #340 "UTF-8 accentuated
characters causing segfault". The patch in this pull request is
also similar.