Avoid calling memcpy with NULL pointers

According to the C/C++ standards, calling `memcpy(NULL, NULL, 0)` is
undefined behaviour. Recent GCC versions may rely on this by optimizing
NULL pointer checks more aggressively, see [1].

This patch tries to avoid calling std::memcpy with zero elements.
As a side effect, explicitly return NULL when requesting an empty block
from MemoryPoolAllocator::Malloc.

This may be related to #301.

[1] https://gcc.gnu.org/gcc-4.9/porting_to.html

Add build status

Add coverall badges

100% line of code coverage

Improve code coverage for iterative parsing

Assert on impossible state transition in Transit(); Put the last case and all non-enumerated cases(also supply assertion for them) in  for code coverage.

Use assertion for impossible case(The Predict() can ensure the token is ColonToken, otherwise it would be marked as Error state. So there is no need to check ColonToken again).