Fix off-by-one on length in wide_string_from_c_string.

[trivial and not reviewed]

The wide string length computed in wide_string_from_c_string is supposed
to include the terminating '\0' but did not, so fixed that. This problem
caused failures with SEGMENT=fast and test/extern/ferguson/c_strings.chpl,
which should be fixed now.



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21973 3a8e244f-b0f2-452b-bcba-4c88e055c3ca