[libunwind][AIX] Call dlclose only when dlsym() fails (#112768)

The personality routine `__xlcxx_personality_v0` in `libc++abi` is
hard-coded in the unwinder as the handler for EH in applications
generated by the legacy IBM C++ compiler. The symbol is resolved
dynamically using `dlopen` to avoid a hard dependency of `libunwind` on
`libc++abi` for cases such as non-C++ applications. However, `dlclose`
was incorrectly called after `dlsym` succeeded, potentially invalidating
the function pointer obtained from `dlsym` when the memory allocated for
the `dlopen` is reclaimed. This PR changes to call `dlclose` only when
`dlsym` fails.

libunwind/src/UnwindCursor.hpp

@@ -2150,9 +2150,9 @@ bool UnwindCursor<A, R>::getInfoFromTBTable(pint_t pc, R &registers) {
             dlsym(libHandle, "__xlcxx_personality_v0"));
         if (xlcPersonalityV0 == NULL) {
           _LIBUNWIND_TRACE_UNWINDING("dlsym() failed with errno=%d\n", errno);
+          dlclose(libHandle);
           assert(0 && "dlsym() failed");
         }
-        dlclose(libHandle);
         errno = saveErrno;
       }
       xlcPersonalityV0InitLock.unlock();