ASAN and UBSAN warnings

[XVilka]

../lib/capn.c:484:17: runtime error: left shift of negative value -1
[       OK ] WireFormat.StructRoundTrip_OneSegment (6 ms)
../lib/capn.c:704:19: runtime error: member access within null pointer of type 'struct copy'
../lib/capn-malloc.c:219:12: runtime error: store to misaligned address 0x7ffcfc967a0a for type 'uint32_t', which requires 4 byte alignment
0x7ffcfc967a0a: note: pointer points here
 00 00  f8 ff ff ff ff ff ff ff  90 7a 96 fc fc 7f 00 00  1e 00 00 00 00 00 00 00  f8 00 00 00 00 00
              ^ 
../lib/capn-malloc.c:220:22: runtime error: store to misaligned address 0x7ffcfc967a0e for type 'uint32_t', which requires 4 byte alignment
0x7ffcfc967a0e: note: pointer points here
 00 00 00 00 ff ff  90 7a 96 fc fc 7f 00 00  1e 00 00 00 00 00 00 00  f8 00 00 00 00 00 00 00  00 00
             ^ 
../lib/capn-malloc.c:225:17: runtime error: store to misaligned address 0x7ffcfc967a0e for type 'uint32_t', which requires 4 byte alignment
0x7ffcfc967a0e: note: pointer points here
 00 00 00 00 00 00  00 00 96 fc fc 7f 00 00  1e 00 00 00 00 00 00 00  f8 00 00 00 00 00 00 00  00 00
             ^