guard short handle string in XsbReader.readHandle

[aizu-m]

Feeding the schema loader a crafted .xsb, it throws out of readHandle:

java.lang.StringIndexOutOfBoundsException: Index 2 out of bounds for length 2
    at java.lang.String.charAt(String.java:1555)
    at org.apache.xmlbeans.impl.schema.XsbReader.readHandle(XsbReader.java:444)

readHandle pulls a component handle off the .xsb string pool and switches on
handle.charAt(0) then handle.charAt(2) without checking the length first. An
empty handle trips charAt(0). A one or two char handle starting with '_' (say
"_X") trips charAt(2), and a three char one walks into QNameHelper.forPretty(handle, 4)
whose substring(4) throws too.

The loader is meant to surface a bad file as SchemaTypeLoaderException, the
reader only wraps IOException, and the method's own unresolved-handle branches
already throw BAD_HANDLE. So this index walk is the odd one out. Spotted it
fuzzing the reader with truncated handle strings.

Guarded the empty handle and the short '_' handle the same way. Valid handles
keep their existing path. Test in XsbReaderHandleTest.