vector: Fix undefined behaviour on realloc

On tests listed bellow the reserve method is being called before any
allocation on the object therefore realloc being called without
any previous allocation. Inside realloc _data is being used with the
operator '[]', as it is nullptr at that moment it's an undefined
behaviour.

This patch simply returns a call to alloc if _data is nullptr.

This tests fails on PowerPC with Segmentation Fault because of this
issue:
segment_vector_array_expsize_assign_exceptions_oom_0_none
segment_vector_array_expsize_assign_exceptions_oom_0_memcheck
segment_vector_array_expsize_capacity_exceptions_oom_0_none
segment_vector_array_expsize_capacity_exceptions_oom_0_memcheck
segment_vector_array_expsize_modifiers_exceptions_oom_0_none
segment_vector_array_expsize_modifiers_exceptions_oom_0_memcheck
segment_vector_vector_expsize_assign_exceptions_oom_0_none
segment_vector_vector_expsize_assign_exceptions_oom_0_memcheck
segment_vector_vector_expsize_capacity_exceptions_oom_0_none
segment_vector_vector_expsize_capacity_exceptions_oom_0_memcheck
segment_vector_vector_expsize_modifiers_exceptions_oom_0_none
segment_vector_vector_expsize_modifiers_exceptions_oom_0_memcheck
segment_vector_vector_fixedsize_assign_exceptions_oom_0_none
segment_vector_vector_fixedsize_assign_exceptions_oom_0_memcheck

Signed-off-by: Lucas A. M. Magalhães <lamm@linux.ibm.com>