Segfault when setting a "utfitem" into a layer

[fblackburn]

Setting the "utfitem" parameter to an attribute that isn't present in any other parameter of the layer cause a segfault.

[fblackburn]

After looking into the code we found out that the variable used to set the size of the item array ("nt") isn't properly increased when the layer contains a "utfitem" parameter in the function "msTokenizeExpression" of "maplayer.c". This problem can cause a buffer overflow if all the used attribute in a layer are different.

[tbonfort]

@rouault the failing test is a side effect of the currently flakey osgeo servers. +1 to merge