Prevent BDCSVD crash caused by index out of bounds.

Antonio Sánchezrequested to merge
cantonios/eigen:bdcsvd into master

For a large matrix of ones, we end up trying to access perm(-1), which causes a memory access error and crash.

Added a basic check for this case, and force zhat to zero, in an attempt to continue. Reports a numerical issue.

Related to #2491 (closed)

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