oleaut32: Add tests for DispGetParam. Fix two issues that crashed the tests.

dlls/oleaut32/dispatch.c

@@ -130,6 +130,13 @@ HRESULT WINAPI DispGetParam(
 
     TRACE("position=%d, cArgs=%d, cNamedArgs=%d\n",
           position, pdispparams->cArgs, pdispparams->cNamedArgs);
+
+    if (pdispparams->cArgs > 0 && !pdispparams->rgvarg)
+        return E_INVALIDARG;
+
+    if (!pvarResult)
+        return E_INVALIDARG;
+
     if (position < pdispparams->cArgs) {
       /* positional arg? */
       pos = pdispparams->cArgs - position - 1;

dlls/oleaut32/tests/Makefile.in

@@ -6,6 +6,7 @@ TESTDLL   = oleaut32.dll
 IMPORTS   = oleaut32 ole32 rpcrt4 user32 gdi32 advapi32 kernel32
 
 CTESTS = \
+	dispatch.c \
 	olefont.c \
 	olepicture.c \
 	safearray.c \