Ensure that length argument of memcmp() isn't seen as negative.

I think this will shut up a weird warning from buildfarm member
serinus.  Perhaps it'd be better to change tsCompareString's
length arguments to unsigned, but that seems more invasive
than is justified.

Part of a general push to remove off-the-beaten-track warnings
where we can easily do so.

src/backend/utils/adt/tsvector_op.c

@@ -1167,7 +1167,7 @@ tsCompareString(char *a, int lena, char *b, int lenb, bool prefix)
 	}
 	else
 	{
-		cmp = memcmp(a, b, Min(lena, lenb));
+		cmp = memcmp(a, b, Min((unsigned int) lena, (unsigned int) lenb));
 
 		if (prefix)
 		{