x11drv: ime: In the update functions if we get a null string with a len report this and fix len to be 0 (Coverity 656 and 655).

Fixes winmm:wave test hanging.

Fixes failing dsound tests in wine.

wininet: Return error codes explicitly from URLCache functions instead of returning a BOOL and storing the error code in the last error value.

This makes the code more efficient and allows errors from functions to
be ignored without side-effects.

The assertion was not valid, because it neglected to take into account
the situation where a line break is forced with a MERF_ENDROW run
(caused by \line control word or pressing Shift-Enter).  This means
that spaces can cause a line wrap after a forced line break as well as
after a paragraph break, so we cannot assert that it is the first row
in the paragraph.

The test for EM_GETLINE was testing to make sure the null terminating
character was written at the end of the text as long as the buffer was
long enough, and also tested to make sure that no other bytes were
written after this null terminating character.  This is consistent with
Windows 2000 and up, but not for previous versions of Windows.

With this patch it's possible to test scripts by running
wine jscript_test.exe.so run <file_name>
Although it's not what Wine tests are for, it proved to be very useful 
for me.