Description:
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When writing phpt tests I was trying to reach a part of the code where the result 
of sqlite_error_string would not return a message.

When passing 127 as an argument to sqlite_error_string I still get a message 
returned though. So this part of the code seems unreachable.

Looking in the function declaration of the extension source the default value for 
error codes is the "Unknown error" message, which also confirms what I already 
assumed. 

Test script:
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var_dump(sqlite_error_string( 127 ) );


Expected result:
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NULL

Actual result:
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string(13) "unknown error"