Adjust allocation code for possible null returns of std.experimental.allocator.
Adjust allocation code for possible null returns of std.experimental.allocator.

etcimon/vibe.d,DBankov/vibe.d,rejectedsoftware/vibe.d,chalucha/vibe.d,gedaiu/vibe.d,Geod24/vibe.d,etcimon/vibe.d