Add missing default for vector types

Each value type has an associated default value, however only numeric types and
reference types are listed, while there are also values of vector types.

Author: ia0

document/core/exec/runtime.rst

@@ -51,11 +51,11 @@ or *external references* pointing to an uninterpreted form of :ref:`extern addre
 .. _default-val:
 
 Each :ref:`value type <syntax-valtype>` has an associated *default value*;
-it is the respective value :math:`0` for :ref:`number types <syntax-numtype>` and null for :ref:`reference types <syntax-reftype>`.
+it is the respective value :math:`0` for :ref:`number types <syntax-numtype>` and :ref:`vector types <syntax-vectype>`, and null for :ref:`reference types <syntax-reftype>`.
 
 .. math::
    \begin{array}{lcl@{\qquad}l}
-   \default_t &=& t{.}\CONST~0 & (\iff t = \numtype) \\
+   \default_t &=& t{.}\CONST~0 & (\iff t = \numtype \vee t = \vectype) \\
    \default_t &=& \REFNULL~t & (\iff t = \reftype) \\
    \end{array}