Fixes the following warnings in `p=server target=release_debug` builds
on Travis CI:
```
./core/os/memory.h: In function 'MainLoop* TestOAHashMap::test()':
./core/os/memory.h:108:111: warning: 'dummy' may be used uninitialized in this function [-Wmaybe-uninitialized]
#define memnew_placement(m_placement, m_class) _post_initialize(new (m_placement, sizeof(m_class), "") m_class)
^
main/tests/test_oa_hash_map.cpp:98:7: note: 'dummy' was declared here
int dummy;
^
scene/resources/bit_mask.cpp:447:3: warning: 'next_i' may be used uninitialized in this function [-Wmaybe-uninitialized]
scene/resources/bit_mask.cpp:448:4: warning: 'next_j' may be used uninitialized in this function [-Wmaybe-uninitialized]
```
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