Check for RAND_bytes failures in the ClientHello.

(Imported from upstream's e1b568dd2462f7cacf98f3d117936c34e2849a6b.)

Our RAND_bytes secretly can't actually fail, but we should propagate the check
upwards.

Change-Id: Ieaaea98dad00bf73b1c0a42c039507d76b10ac78
Reviewed-on: https://boringssl-review.googlesource.com/4003
Reviewed-by: Adam Langley <agl@google.com>