vbscript: Fix handling null argument in CStr.

dlls/vbscript/global.c

@@ -528,6 +528,9 @@ static HRESULT Global_CStr(vbdisp_t *This, VARIANT *arg, unsigned args_cnt, VARI
 
     TRACE("%s\n", debugstr_variant(arg));
 
+    if(V_VT(arg) == VT_NULL)
+        return MAKE_VBSERROR(VBSE_ILLEGAL_NULL_USE);
+
     hres = to_string(arg, &str);
     if(FAILED(hres))
         return hres;

dlls/vbscript/tests/api.vbs

@@ -129,6 +129,14 @@ TestCStr 3, "3"
 if isEnglishLang then TestCStr 3.5, "3.5"
 if isEnglishLang then TestCStr true, "True"
 
+sub testCStrError()
+    on error resume next
+    Error.clear()
+    CStr(null)
+    call ok(Err.number = 94, "Err.number = " & Err.number)
+end sub
+call testCStrError()
+
 Call ok(getVT(Chr(120)) = "VT_BSTR", "getVT(Chr(120)) = " & getVT(Chr(120)))
 Call ok(getVT(Chr(255)) = "VT_BSTR", "getVT(Chr(255)) = " & getVT(Chr(255)))
 Call ok(Chr(120) = "x", "Chr(120) = " & Chr(120))