oss-fuzz: Add allocation failure fuzzing via nalloc

Integrate nalloc fuzzing to systematically test error handling paths
when memory allocations fail. This technique previously found a memory
leak in pCAL chunk handling and is already in production use via
OSS-Fuzz on libwebp and libssh.

This is enabled via the `_nalloc` target suffix or the `NALLOC_FREQ`
environment variable.

Reviewed-by: Cosmin Truta <ctruta@gmail.com>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>