Guard against sentry initialization mid sqlalchemy cursor

[apmorton]

In an application that is executing sqlalchemy cursors on a thread concurrently with another thread calling sentry_sdk.init it is possible for _after_cursor_execute to be called on a cursor that never had _before_cursor_execute called.

  File "site-packages/sentry_sdk/integrations/sqlalchemy.py", line 98, in _after_cursor_execute
    span = context._sentry_sql_span
           ^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'PGExecutionContext_psycopg2' object has no attribute '_sentry_sql_span'

[sentrivana]

LGTM, thank you @apmorton!