Avoid static analyzer warnings regarding uninitialized arguments

Avoid static analyzer warnings regarding uninitialized arguments

Avoid static analyzer warnings regarding "Function call argument
is a pointer to uninitialized value" in cases where we are
intentionally using such arguments.

This is achieved by using ...

`f(b.begin(), b.end())` (`std::array<char, N>`)

... instead of ...

`f(b, b + N)` (`char b[N]`)

Rationale:
* Reduce false positives by guiding static analyzers regarding our
  intentions.

Before this commit:

```
$ clang-tidy-3.5 -checks=* src/bench/base58.cpp
bench/base58.cpp:23:9: warning: Function call argument is a pointer to uninitialized value [clang-analyzer-core.CallAndMessage]
        EncodeBase58(b, b + 32);
        ^
$ clang-tidy-3.5 -checks=* src/bench/verify_script.cpp
bench/verify_script.cpp:59:5: warning: Function call argument is a pointer to uninitialized value [clang-analyzer-core.CallAndMessage]
    key.Set(vchKey, vchKey + 32, false);
    ^
$
```

After this commit:

```
$ clang-tidy-3.5 -checks=* src/bench/base58.cpp
$ clang-tidy-3.5 -checks=* src/bench/verify_script.cpp
$
```