savepng: fix a null pointer dereference

spotted by coverity, also I reproduced it once with command like
`wesnoth --render-image foo.png`
addon/mg: Drop superfluous NULL pointer check

update_all_button's value is obtained from operator new, which will
throw if the allocation fails, so it can never be NULL in subsequent
statements.

Found by coverity.
Add an assertion check to detect a null pointer case

Also preemptively block subsequent dereferences.

Hopefully somebody will eventually hit an assertion failure here and
report back to help me understand how such a situation could possibly
arise, so I can decide whether to remove the assert and leave the new
loop conditional intact or change it to match the intended behavior
(e.g. it's possible `!active || active->empty()` would be preferable in
some scenario).

Spotted by Coverity.
gamestate_inspector: add a ctor initialization for lua_button ptr

This was a minor oversight by me, issue raised by Coverity.
add assertions after many unchecked dynamic casts

These were all reported by coverity, and indeed seem to have been
assumed to work without failing at runtime.
fix a bug that caused std::hex to get written into std::cerr

reported by gfgtdf
travis: add a dedicated build for translations
travis: disable all tests on the translations build
move default_generate_map to its own file
move a rand() call
add seed to default_generate_map
add seed to map_generator
use mt19937 instead of simple_rng in cavegen.
add seed textbox in map editor random generation
make editor search for scenario_generation too
fixup build
Update CMakeLists.txt
fs: Don't complain about failing to read /proc/self/exe with Boost.filesystem

This is intended to silently fail on Unix-like platforms where no procfs
is available, and indeed that's how it works in the non-BFS
implementation of get_exe_dir(). The caller (currently a single function
in game.cpp/wesnoth.cpp) must be able to deal with this situation
in a graceful fashion.
add bindings for map_location functions to the lua kernel base

I mainly want to use this for random map generators, it would be
silly to rewrite this stuff in lua after we already have it in C++
(and unit tested there).
Merge branch 'master' of https://github.com/wesnoth/wesnoth