Fix random dropping of characters on dumping ASCII encoded XML

Fix a bug in xmlCharEncOutput return value which will cause
xmlNodeDumpOutput to drop characters randomly.

xmlCharEncOutput returns zero if the length of the input buffer is
zero but ignores the fact that it may already encoded the input buffer
and the input's length is zero due to the fact that xmlEncOutputChunk
returned -2 errors and underlying code tries to fix the error by
encoding the input.

xmlCharEncOutput is collecting the number of bytes written to the
output buffer but is returning zero instead of the total number of
bytes in this situation. This commit will fix this issue by returning
the total number of bytes instead. So the xmlNodeDumpOutput will also
continue writing and will not stop due to the fact that it mistakenly
thinks the output buffer is not changed in that iteration.

Fixes #314
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