Fix mis-sized allocation in x509_vpm.c

Caught by clang scan-build. (The allocation was larger than it should have
been.)

Change-Id: Ideb800118f65aaba1ee85b7611c8a705671025a8
Reviewed-on: https://boringssl-review.googlesource.com/1340
Reviewed-by: Adam Langley <agl@google.com>