...so that e.g.,
DateAdd("x", 1, "1/31/2004")
(where "x" is not in the list of valid values for the Add parameter, "yyyy",
"q", "m", etc.) leads to a Basic runtime error rather than going into a
seemingly endless while(nNewMonth>nTargetMonth) loop at the end of
RTLFUNC(DateAdd) (basic/source/runtime/methods.cxx).
Change-Id: I15c3bdb62723ffddf36ff2396ffb294369d93ff8