Preliminary fix for #225
Preliminary fix for #225

lp do not work at the moment, because mg_fopen resets unused outputs
of the struct file. In this fix, the required size member is copied
from the previous call to mg_stat.

However, the real maintenance problem is, operations like mg_fopen
and mg_stat have to do exactly and exclusively what their names
suggest. They should not set different members of a common output
structure (struct file).
But this will be a more comprehensive change which needs more time.

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