fixed compilation with assertions disabled:
fixed compilation with assertions disabled:

variable "msg" is only used in an "assert()" call;
hence we better ensure that is is also declared
only if assertions are enabled;
this prevents
"
sql/src/backends/monet5/sql_optimizer.mx(337): error #177: variable "msg" was declared but never referenced
  str msg = getContext(cntxt,mb, &m, NULL);
      ^
"

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb