coverity: hint git_vector_foreach does not deref NULL contents

coverity: hint git_vector_foreach does not deref NULL contents

Coverity does not comprehend the connection between a vector's
size and the contents pointer, that is that the vector's pointer
is non-NULL when its size is positive. As the vector code should
be reasonably well tested and users are expected to not manually
modify a vector's contents it seems save to assume that the
macros will never dereference a NULL pointer.

Fix Coverity warnings by overriding the foreach macros with
macros that explicitly aborting when (v)->contents is NULL.