[[ Bug 13695 ]] MCU_fix_path: Correctly handle '..' as 2nd relpath component.

MCU_fix_path() previously implemented the following transformations:

   /../suffix           --> /suffix       [correct]
   prefix/xxx/../suffix --> prefix/suffix [correct]
   xxx/../suffix        --> /suffix       [incorrect]

This problem only occurred when the path component cancelled by a '..'
path component occurred as the first element in a relative path.

This patch corrects the handling by ensuring that when a second path
separator cannot be located before the '..' component, one is removed
from after it instead.
Merge pull request #1551 from peter-b/bugfix-13695

[[ Bug 13695 ]] MCU_fix_path: Correctly handle '..' as 2nd relpath component