shared/specifier: be less extravagant with memory allocations

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.