Fix two memcpy() bugs in the new injection point code

Fix two memcpy() bugs in the new injection point code

1. The memcpy()s in InjectionPointAttach() would copy garbage from
beyond the end of input string to the buffer in shared memory. You
won't usually notice, but if there is not enough valid mapped memory
beyond the end of the string, the read of unmapped memory will
segfault. This was flagged by the Cirrus CI build with address
sanitizer enabled.

2. The memcpy() in injection_point_cache_add() failed to copy the NULL
terminator.

Discussion: https://www.postgresql.org/message-id/0615a424-b726-4157-afa7-4245629f9512%40iki.fi