Fix invalid memory access in CScript::operator+=

[sickpig]

This is a port of bitcoin/bitcoin/pull/11284. The following is the original PR discussion:

This is a fix for #11114 -- invoking "s += s" gets turned into "s.insert(s.end(), s.begin(), s.end())" which can result in an invalid memory access is s.capacity() < 2*s.size() (because s gets resized and possibly moved, so s.begin() and s.end() become invalid references when reading the values to be appended).
    
The fix is straightforward: reserve enough space in advance, so that insert() doesn't need to resize and thus its arguments remain valid.
    
A simple test case is added as well; though you probably need to run it via valgrind to actually catch the problem when it's not fixed...

[sickpig]

For some reason Travis didn't trigger a travis run. will investigate.