log: Prevent getting time if it's unnecessary

log: Prevent getting time if it's unnecessary

Small performance gain:
name              old time/op    new time/op    delta
Itoa-4              95.4ns ± 4%    95.6ns ± 3%    ~     (p=0.256 n=45+46)
Println-4            480ns ± 4%     476ns ± 5%  -0.87%  (p=0.003 n=45+45)
PrintlnNoFlags-4     316ns ± 3%     299ns ± 4%  -5.38%  (p=0.000 n=42+44)

name              old alloc/op   new alloc/op   delta
Itoa-4               0.00B          0.00B         ~     (all equal)
Println-4            21.0B ± 0%     21.0B ± 0%    ~     (all equal)
PrintlnNoFlags-4     21.0B ± 0%     21.0B ± 0%    ~     (all equal)

name              old allocs/op  new allocs/op  delta
Itoa-4                0.00           0.00         ~     (all equal)
Println-4             2.00 ± 0%      2.00 ± 0%    ~     (all equal)
PrintlnNoFlags-4      2.00 ± 0%      2.00 ± 0%    ~     (all equal)

Change-Id: Idcd03609a5a437a69ffa7004a673bf0b8d22e7ad
Reviewed-on: https://go-review.googlesource.com/38056
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>