Avoid reading before the buffer start when generating errors from require.

Avoid reading before the buffer start when generating errors from require.

In pp_require, the error reporting code treats file names ending /\.p?h\z/
specially. The detection code for this, as refactored in 2010 by commit
686c4ca09cf9d6ae, could read one or two bytes before the start of the
filename for filenames less than 3 bytes long. (Note this cannot happen with
module names given to use or require, as appending ".pm" will always make the
filename at least 3 bytes long.)