util/charset/convert: do not pretend to realloc

util/charset/convert: do not pretend to realloc

It seems very likely that our clever attempts to dynamically realloc
the output buffer were never triggered. Two lines of reasoning lead to
this conclusion:

1. We allocate 3 * srclen to start with, but no conversion we use will
   more than that. To be precise, from 8-bit charsets we will only deal
   with codepoints in the Unicode basic multilingual plane (up to 0xFFFF).

   These can all be expressed as 3 or fewer utf-8 bytes. In UTF16 they
   are naturally 2 bytes, while in the DOS codes they are 1 byte.

   We have checked the code tables, and can not find a plausible
   (e.g. not EBCDIC) DOS code page or unix charset that is outside
   this range.  Clients cannot chose the code page, the only code
   pages we will use come from 'unix charset' and 'dos charset'
   smb.conf parameters.

   Therefore the worst that can possibly happen is we expand 1 byte into 3
   (specifically, when converting some e.g. CP850 codepoints to UTF-8).

2. If the reallocation was ever used, the results would have been
   catastrophically wrong, as the input pointer was not reset.

Therefore we skip the complication of the goto loop and let E2BIG be
just another impossible error to report.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>