winmm: mixerGetLineInfoA wrapper shouldn't convert strings on failure

[Dan Kegel]

[Yet another post-1.0 patch.  I'll resend them after 1.0 is out.]

If the call fails, the strings aren't set, so don't try
to convert them.
Added a test to verify this for at least the MMSYSERR_INVALFLAG
case, and made sure it passed on XP.

Fixes 27 valgrind warnings like

Conditional jump or move depends on uninitialised value(s)
   at strlenW (unicode.h:212)
   by WideCharToMultiByte (locale.c:1902)
   by mixerGetLineInfoA (winmm.c:664)
   by mixer_test_deviceA (mixer.c:398)
 Uninitialised value was created by a stack allocation
   at mixerGetLineInfoA (winmm.c:617)
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