user32: GetTabbedTextExtent() returns non-zero value when nCount == 0.

dlls/user32/tests/text.c

@@ -617,6 +617,9 @@ static void test_TabbedText(void)
     ret = GetTextMetricsA( hdc, &tm);
     ok( ret, "GetTextMetrics error %u\n", GetLastError());
 
+    extent = GetTabbedTextExtentA( hdc, "x", 0, 1, tabs);
+    ok( extent == 0, "GetTabbedTextExtentA returned non-zero on nCount == 0\n");
+
     extent = GetTabbedTextExtentA( hdc, "x", 1, 1, tabs);
     cx = LOWORD( extent);
     cy = HIWORD( extent);

dlls/user32/text.c

@@ -1281,6 +1281,8 @@ static LONG TEXT_TabbedTextOut( HDC hdc, INT x, INT y, LPCWSTR lpstr,
     int i, j;
     int start = x;
 
+    if (!lpstr || count == 0) return 0;
+
     if (!lpTabPos)
         cTabStops=0;