other crashes

Some string manipulating SQL functions use a shared string object intended to
contain an immutable empty string. This object was used by the SQL function
SUBSTRING_INDEX() to return an empty string when one argument was of the wrong
datatype. If the string object was then modified by the sql function INSERT(),
undefined behavior ensued.

Fixed by instead modifying the string object representing the function's
result value whenever string manipulating SQL functions return an empty
string.

Relevant code has also been documented.