The vector table must be 256 aligned to work reliably, it must have been working by accident up to now. This modification tested on a u-blox c027 board. Without the change the value of `SCB->VTOR` and int_table are as follows:
`SCB->VTOR 0x10000280, int_table 0x100002a0`
...with the change they are as follows:
`SCB->VTOR 0x10000400, int_table 0x10000400`
...and the test passes.
See issue:
https://github.com/mbedmicro/mbed/issues/992