resolved all microsoft native recommened warnings

[matthewhelyar]

Resolved all the Microsoft Native Recommended warnings. Compiled using C++14 on MSVC, but should be backwards compatible further than that. Wasn't sure what version to target.

• default initialized uninitialized variables.
• replaced bitwise & which was used to combine bools with logical &&. Better represents intent and allows short-circuiting, which bitwise & does not.
• added a null check
• replaced union type punning with memcpy to avoid undefined behavior.
• Ensure null termination of mantissa_buffer for safe use of strlen
• Some explicit casting to avoid risk of arithmetic overflow.

[matthewhelyar]

I can't get the default initialization of arrays to work in the AppVeyor tests on Visual Studio version 14 or below as the warnings about default initialization of T = char and and T = wchar_t are treated as errors in those tests. (Talk about irony...) All other CI pass. You are welcome to all the other fixes, but the uninitialized variables are the source of most of the warnings I was trying to resolve. I've therefore closed the PR.

6fbb5c0 passed the most tests