guard non-positive requested_digits in DigitGenCounted

[Ramya-9353]

ASan (Release build), FastDtoa(1234567.0, FAST_DTOA_PRECISION, 0, ...) into a one-byte buffer:

==ERROR: AddressSanitizer: heap-buffer-overflow
WRITE of size 1 ... 0 bytes after 1-byte region
    #0 double_conversion::FastDtoa(...) fast-dtoa.cc  (via DigitGenCounted)

DigitGenCounted's integral loop decrements requested_digits and only breaks on an exact 0, so a call with 0 (the first decrement makes it -1) never stops and emits every digit of the value past the end of the buffer; a negative count behaves the same way. DoubleToAscii special-cases PRECISION with 0 digits before it ever reaches here, and #306 added the matching guard to the Bignum slow path in GenerateCountedDigits, but the Grisu3 fast path, reachable through the installed FastDtoa API, was still unguarded.

Fix returns false with length 0 when requested_digits <= 0, before the loop runs, so nothing is written. Valid precision requests are unchanged. The regression test fences a one-byte buffer with a sentinel and checks it survives; the full ctest suite passes in Debug and Release.

[floitsch]

LGTM.
Thanks.