stop ExtendedMessageFormat seekNonWs reading past the pattern end

[dxbjavid]

The registry-based ExtendedMessageFormat constructor parses its pattern with seekNonWs, which asks the split matcher about the character at the current parse index without first checking it is still inside the buffer. A truncated format element such as {, {0, or trailing text like {0}extra{ leaves the parse position at the end of the pattern, so the matcher reads one past the char array and the constructor throws ArrayIndexOutOfBoundsException rather than the documented IllegalArgumentException that the plain constructor already gives for the same input. This bounds the seek to the pattern length so those patterns report the usual unterminated-format-element error; a regression test covers the three shapes above.