Defensively cap number_of_leading_zeros in getNationalSignificantNumber

[ghaithabdulreda]

Cap the number_of_leading_zeros field at 3 in getNationalSignificantNumber()
for both Java and C++ to prevent Out-of-Memory DoS via malicious protobuf
input. The field has no upper bound in the protobuf definition and is used
directly for array allocation.

Bug: https://issuetracker.google.com/issues/523402884

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[ghaithabdulreda]

@google-cla check

[ghaithabdulreda]

Hi, just a friendly reminder. Could anyone from the team take a look at this fix when you have a moment?

[ghaithabdulreda]

@rohininidhi @kkeshava Gentle ping on this fix. Could either of you take a look? It's been two weeks.

[mandlil]

Hi @ghaithabdulreda
Thanks for the PR! Bumping the limit from 3 to 10 would be safer. 3 is already a valid count today, so setting it to 10 will prevent future code changes if new numbers use more, while still protecting against OOM.

[ghaithabdulreda]

@mandlil Thanks for the review! I’ve updated the cap from 3 to 10 in both the Java and C++ implementations, as suggested.