[[ Valgrind ]] Ensure MCStack members get properly initialised.

Prevents use of uninitialised values.
libfoundation: MCStringCreateMutable(): Set length of string to zero.

When MCStringCreateMutable() is called with a non-zero
p_initial_capacity, __MCStringExpandAt() provides a buffer of the
requested length for the __MCString.  However, it also sets the
char_count of the __MCString to the same length.  This can cause the
resulting __MCString to contain arbitrary data.
Merge branches 'misc-mutablestrings' and 'valgrind-mcstack' into develop-7.0

Closes #1432 and #1433.