rsyslog mmpstrucdata stack overflow

The issue occurs while mmpstrucdata parses RFC 5424
structured-data parameter values.

In affected versions, parseSD_PARAM() allocates a fixed
32,768-byte stack buffer:

uchar pVal[32 * 1024];

It then passes this buffer to parsePARAM_VALUE() without
providing its size.

parsePARAM_VALUE() writes the parameter value into the
buffer without checking the destination size. Its loop is
bounded only by the length of the structured-data input.

If rsyslog accepts a structured-data parameter value larger
than the fixed buffer, attacker-controlled data can overwrite
the stack.