This fixes following tests under UBSAN:
vector_modifiers_type_requirements_0_none
vector_parameters_0_none

They failed with following error message:

"runtime error: reference binding to null pointer of type"

This fix is continuation of 41ddc88a

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>

Merge stable 1.7 into stable 1.8

Merge stable-1.6 into stable-1.7

docker: make build- and push-image scripts more verbose

Merge stable-1.5 into stable-1.6

otherwise it will still use found (most likely, newer) clang-format.

Update doxygen settings

bump docker images versions

and set clang-format to fixed version (6.0).

fixes #854

Merge stable 1.7 into stable 1.8

Merge stable-1.6 into stable-1.7

Merge stable 1.5 into stable 1.6

contigous_iterator: make operator[] take signed integral instead

of unsigned.

The reason is that Windows cannot decide for a call like this:

`it[0] = 10`

whether to use:
* operator[](unsigned) -- one conversion from signed to unsigned
* operator T*() and then build in operator[](signed) -- one conversion

Merge stable 1.7 to stable 1.8

Merge stable-1.6 into stable-1.7

Merge stable-1.5 into stable-1.6

fix dir creation in auto docs update

tests: cleanup source of removed test

Only parse libpmemobj version if it matches a pattern (major[.]minor.*)

doc: enable auto gh-pages generation in docker script

Ref: https://github.com/pmem/libpmemobj-cpp/issues/377

Add skip flags for additional artifacts built/installed in dockerimages

insert_erase_mt was removed but it's source stayed.