fix compile issue: missing braces around initializer

[bitkevin]

If you compile with gcc(version 4.8.2), it'll cause failure. Error is "missing braces around initializer".

[gwillen]

This appears to be due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119 . I don't think removing the initializer is a correct workaround, because it means the struct will not be initialized at all (in my understanding.)

I see several options:

• If the zero-initializer is unnecessary, because the variable is never read from before assignment, it could be safe to remove it.
• The warning could be silenced by changing the initializer to one that satisfies this version of gcc; I speculate this could be done by using a longer initializer that lists all the fields and zeroes them individually.
• The struct could be zeroed with a memset or equivalent (the bug filer describes this as "common but nonportable"; I don't know how nonportable that is.)
• The struct could be zeroed using a trick described in the bug (creating a static const object of the same type, and assigning from that.)

[rustyrussell]

His fix is correct; blkhash is in fact never used unless fully set. I'll apply it now, thanks!