richedit: Handle negative position given to EM_POSFROMCHAR. (Bug 13672)

[Dylan Smith]

This case would cause an assertion failure in ME_RunOfsFromCharOfs, and
the included test indicates it should be used as if it were position 0.
This seems to be the only place where ME_RunOfsFromCharOfs could receive
a negative position, so it makes sense to handle this case just on
EM_POSFROMCHAR messages.
---
 dlls/riched20/editor.c       |    1 +
 dlls/riched20/tests/editor.c |    4 ++++
 2 files changed, 5 insertions(+), 0 deletions(-)
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