[acc-warning-fix][assignment-operator-unchecked-source]

"operator= does not have a check for the source and destination
addresses being non-identical"
If the source and destination strings point to the same place, then this
will result in undefined behavior (delete[] the ptr and then memcpy()
from it).
Merge pull request #513 from tpaviot/review/aCC-LDOMBasicString-fix

Fix LDOMBasicString assignment operator