TOOLBAR_InsertButtonA: If iString==-1 then don't use it as a pointer.

diff --git a/dlls/comctl32/toolbar.c b/dlls/comctl32/toolbar.c
index c6aaa88..9a44d3b 100644
--- a/dlls/comctl32/toolbar.c
+++ b/dlls/comctl32/toolbar.c
@@ -2785,7 +2785,8 @@
        int	len;
        LPSTR	ptr;
 
-       if(lpTbb->iString) {
+       /* FIXME: iString == -1 is undocumented */
+       if(lpTbb->iString && lpTbb->iString!=-1) {
            len = strlen((char*)lpTbb->iString) + 2;
            ptr = COMCTL32_Alloc(len);
            nIndex = infoPtr->nNumButtons;