Add missing va_end() call when the log is full

No need to go out of memory, too ;)

cppcheck reported:
[rockbox/firmware/logf.c:338]: (error) va_list 'ap' was opened but not closed by va_end().

Change-Id: I00e4c04d7e3d5d1415aa5066487ce1d9209e53aa

firmware/logf.c

@@ -335,6 +335,7 @@ void _logdiskf(const char* file, const char level, const char *fmt, ...)
     {
         strcpy(&logdiskfbuffer[logdiskfindex-8], "LOGFULL");
         logdiskfindex=MAX_LOGDISKF_SIZE;
+        va_end(ap);
         return;
     }