Do string literals that end with a null-terminator contain an extra null-terminator?
All string literals have an implicit null-terminator, irrespective of the content of the string.
The standard (6.4.5 String Literals) says:
A byte or code of value zero is appended to each multibyte character sequence that results from a string literal or literals.
So, the string literal "abc\0" contains the implicit null-terminator, in addition to the explicit one. So, the array a contains 5 elements.