[Rd] strsignif.c, util.c (PR#10635)

[A.R.Runnalls at kent.ac.uk]

In R 2.6.1, a couple of places (discovered using valgrind) where the
requested size of string buffers fails to account correctly for the
trailing null byte:

1. In src/appl/strsignif.c, 'f0' and 'form' at l. 108-9 each need at
least 1 extra byte.

2. In src/main/util.c, 'out' at l. 1081 needs at least one extra byte.

(Remember that the return value of strlen does not include the null byte.)

Andrew Runnalls