shared/specifier: be less extravagant with memory allocations
ubsan times out because we do too many allocations:
$ valgrind build/fuzz-unit-file test/fuzz-regressions/fuzz-unit-file/oss-fuzz-6977-full
...
test/fuzz-regressions/fuzz-unit-file/oss-fuzz-6977-full... ok
==1757==
==1757== HEAP SUMMARY:
==1757== in use at exit: 0 bytes in 0 blocks
==1757== total heap usage: 199,997 allocs, 199,997 frees, 90,045,318,585 bytes allocated
...
==3256== total heap usage: 100,120 allocs, 100,120 frees, 13,097,140 bytes allocated
https://oss-fuzz.com/v2/issue/
4651449704251392/6977 should now be really fixed.
e3c3d6761b3e7d was the first attempt, but even with this change,
e3c3d6761b3e7d
still makes sense.