Fix NULL pointer dereference in ResolveAddress()

[hirorogo]

Summary

Both overloads of ResolveAddress() in src/sofa/pbrpc/rpc_endpoint.cc accept an RpcEndpoint* endpoint parameter but dereference it unconditionally (*endpoint = it->endpoint()) without checking for NULL. Added NULL validation with error logging, returning false early if endpoint is NULL.

PoC

// Reproduction: ResolveAddress NULL pointer dereference
// When endpoint parameter is NULL, *endpoint = it->endpoint() crashes.
#include <cstdio>
#include <cstring>

struct RpcEndpoint { char host[64]; int port; };

// Simulates the buggy ResolveAddress
bool ResolveAddress_buggy(const char* host, const char* svc, RpcEndpoint* endpoint) {
    // Simulate successful DNS resolution
    RpcEndpoint resolved;
    strncpy(resolved.host, host, 63);
    resolved.port = 8080;

    // BUG: no NULL check
    *endpoint = resolved;  // CRASH when endpoint==NULL
    return true;
}

bool ResolveAddress_fixed(const char* host, const char* svc, RpcEndpoint* endpoint) {
    if (endpoint == NULL) {
        printf("  FIXED: endpoint is NULL, returning false\n");
        return false;
    }
    RpcEndpoint resolved;
    strncpy(resolved.host, host, 63);
    resolved.port = 8080;
    *endpoint = resolved;
    return true;
}

int main() {
    printf("=== FIXED ===\n");
    ResolveAddress_fixed("localhost", "8080", NULL);

    printf("=== BUGGY ===\n");
#ifdef _WIN32
    __try {
        ResolveAddress_buggy("localhost", "8080", NULL);
    } __except(1) {
        printf("  ACCESS VIOLATION: NULL pointer dereference confirmed\n");
    }
#endif
    return 0;
}

Output (MSVC /EHa):

=== FIXED ===
  FIXED: endpoint is NULL, returning false
=== BUGGY ===
  ACCESS VIOLATION: NULL pointer dereference confirmed

Test plan

• Existing tests still pass
• Calling ResolveAddress() with NULL endpoint returns false instead of crashing

[hirorogo]

Closing in favor of #252 which has a cleaner diff. Same fix.