Stricter test in String::from_codepoint regarding invalid codepoints.
Stricter test in String::from_codepoint regarding invalid codepoints.

Checking if ONIGENC_CODE_TO_MBC(...) <= 0 is not enough in discovering
if we're trying to convert an invalid codepoint into a string.
Using Encoding::precise_mbclen ensures the problem is caught and raised.

pH14/rubinius,Azizou/rubinius,ruipserra/rubinius,heftig/rubinius,ruipserra/rubinius,sferik/rubinius,sferik/rubinius,pH14/rubinius,kachick/rubinius,ngpestelos/rubinius,ruipserra/rubinius,digitalextremist/rubinius,jsyeo/rubinius,benlovell/rubinius,heftig/rubinius,jemc/rubinius,Azizou/rubinius,Wirachmat/rubinius,kachick/rubinius,jemc/rubinius,jemc/rubinius,jsyeo/rubinius,sferik/rubinius,dblock/rubinius,Wirachmat/rubinius,mlarraz/rubinius,pH14/rubinius,ruipserra/rubinius,jsyeo/rubinius,jemc/rubinius,digitalextremist/rubinius,Wirachmat/rubinius,kachick/rubinius,benlovell/rubinius,Wirachmat/rubinius,benlovell/rubinius,dblock/rubinius,kachick/rubinius,benlovell/rubinius,pH14/rubinius,digitalextremist/rubinius,dblock/rubinius,Wirachmat/rubinius,pH14/rubinius,Azizou/rubinius,heftig/rubinius,sferik/rubinius,ngpestelos/rubinius,jemc/rubinius,ngpestelos/rubinius,sferik/rubinius,mlarraz/rubinius,pH14/rubinius,dblock/rubinius,kachick/rubinius,dblock/rubinius,ngpestelos/rubinius,heftig/rubinius,pH14/rubinius,sferik/rubinius,Azizou/rubinius,Wirachmat/rubinius,digitalextremist/rubinius,jsyeo/rubinius,mlarraz/rubinius,benlovell/rubinius,ngpestelos/rubinius,jsyeo/rubinius,mlarraz/rubinius,kachick/rubinius,jsyeo/rubinius,Wirachmat/rubinius,heftig/rubinius,mlarraz/rubinius,Azizou/rubinius,sferik/rubinius,kachick/rubinius,digitalextremist/rubinius,digitalextremist/rubinius,ruipserra/rubinius,ruipserra/rubinius,benlovell/rubinius,jemc/rubinius,jsyeo/rubinius,ruipserra/rubinius,mlarraz/rubinius,ngpestelos/rubinius,dblock/rubinius,ngpestelos/rubinius,heftig/rubinius,mlarraz/rubinius,jemc/rubinius,dblock/rubinius,Azizou/rubinius,benlovell/rubinius,Azizou/rubinius,digitalextremist/rubinius,heftig/rubinius,kachick/rubinius