Fix concat not 0-terminating when String shrunk (#4962)
Fix concat not 0-terminating when String shrunk (#4962)

As @devyte noticed, PR #4955 has an issue when you catenate a string to
itself and the string used to hold a longer value because it does not
explicitly 0-terminate the resulting string.  If the string was extended,
however, reserve() would 0-terminate by default.

Always terminate the result of `s += s;` now.
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