SRombauts / SQLiteCpp / 28444563093

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Guard null C pointers in Exception, Database, and Statement against UB (#552)

Fixes a cluster of null C-pointer bugs found in the code review. Each
one builds a std::string (or std::runtime_error) from a SQLite C API
pointer that can legitimately be null, which is undefined behavior.

- `Statement::getExpandedSQL()` now returns an empty string instead of
constructing a `std::string` from a null `sqlite3_expanded_sql()` (null
on out of memory, over `SQLITE_LIMIT_LENGTH`, or in `SQLITE_OMIT_TRACE`
builds, where it always returns null). Empty is used rather than
throwing because this is a diagnostic/logging helper that is often
called from error-handling paths, and an `SQLITE_OMIT_TRACE` build would
otherwise throw on every call.
- `Statement::getColumnIndex()` skips a null `sqlite3_column_name()`
when building the name map.
- `Exception(const char*, int)` falls back to an empty message when
passed null.
- `Database(const char*)` falls back to an empty filename when passed
null, which matches how SQLite treats a null filename (a private
temporary database).

Adds regression tests for the two paths reachable from the public API: a
null Exception message and a null Database filename. The two Statement
paths need a null from inside SQLite (allocation failure or an
OMIT_TRACE build) so they are not unit-testable here; the guards are the
fix.

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