small error in FastConcat3.diff

[Bert Wesarg]

Hi Tony,

I found a small error in your code from the patch. The function
concatenateNwithSep() returns either the length of the string, or -1
for error. But the POP() macro can also return an error (STAT_ERROR ==
4) if the stack underflows. On the caller site you check only for <0,
but this catches not the underflow error.

Regards
Bert