evp: prevent underflow in base64 decoding

This patch resolves RT ticket #2608.

(Imported from upstream's d0666f289ac013094bbbf547bfbcd616199b7d2d.)

Change-Id: If0c8f583f3f643f3db6b6158cbc9ad7541bab8ec
Reviewed-on: https://boringssl-review.googlesource.com/4060
Reviewed-by: Adam Langley <agl@google.com>