sd-event: check the allocation before calling expand_to_usable()

As it might hide a possible allocation error since it uses
the returns_nonnull attribute:

AddressSanitizer:DEADLYSIGNAL
=================================================================
==8==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000004 (pc 0x7f317897db8d bp 0x7ffd436fe9a0 sp 0x7ffd436fe970 T0)
==8==The signal is caused by a WRITE memory access.
==8==Hint: address points to the zero page.
SCARINESS: 10 (null-deref)
    #0 0x7f317897db8d in source_new /work/build/../../src/systemd/src/libsystemd/sd-event/sd-event.c:1214:18
    #1 0x7f317897e68c in sd_event_add_time /work/build/../../src/systemd/src/libsystemd/sd-event/sd-event.c:1417:13
    #2 0x7f317897a0f6 in event_reset_time /work/build/../../src/systemd/src/libsystemd/sd-event/event-util.c:68:21
    #3 0x4e2c8e in client_initialize_time_events /work/build/../../src/systemd/src/libsystemd-network/sd-dhcp-client.c:1366:13
    #4 0x4eb0fd in client_initialize_events /work/build/../../src/systemd/src/libsystemd-network/sd-dhcp-client.c:1380:9
    #5 0x4eb0fd in client_start_delayed /work/build/../../src/systemd/src/libsystemd-network/sd-dhcp-client.c:1410:16
    #6 0x4e30aa in client_start /work/build/../../src/systemd/src/libsystemd-network/sd-dhcp-client.c:1415:16
    #7 0x4e30aa in sd_dhcp_client_start /work/build/../../src/systemd/src/libsystemd-network/sd-dhcp-client.c:2045:13
    #8 0x4e700e in LLVMFuzzerTestOneInput /work/build/../../src/systemd/src/libsystemd-network/fuzz-dhcp-client.c:73:15
    #9 0x5062f8 in NaloFuzzerTestOneInput (/build/fuzz-dhcp-client+0x5062f8)
    #10 0x525283 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
    #11 0x524a6a in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:514:3
    #12 0x526139 in fuzzer::Fuzzer::MutateAndTestOne() /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:757:19
    #13 0x526e05 in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector<fuzzer::SizedFile, std::__Fuzzer::allocator<fuzzer::SizedFile> >&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:895:5
    #14 0x51616f in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:912:6
    #15 0x516a38 in LLVMFuzzerRunDriver /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:925:10
    #16 0x506555 in main (/build/fuzz-dhcp-client+0x506555)
    #17 0x7f3177ce3082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082) (BuildId: 1878e6b475720c7c51969e69ab2d276fae6d1dee)
    #18 0x420c4d in _start (/build/fuzz-dhcp-client+0x420c4d)

Found by Nallocfuzz.

src/libsystemd/sd-event/sd-event.c

@@ -1203,11 +1203,12 @@ static sd_event_source *source_new(sd_event *e, bool floating, EventSourceType t
         assert(type < _SOURCE_EVENT_SOURCE_TYPE_MAX);
         assert(size_table[type] > 0);
 
-        /* We use expand_to_usable() here to tell gcc that it should consider this an object of the full
-         * size, even if we only allocate the initial part we need. */
-        s = expand_to_usable(malloc0(size_table[type]), sizeof(sd_event_source));
+        s = malloc0(size_table[type]);
         if (!s)
                 return NULL;
+        /* We use expand_to_usable() here to tell gcc that it should consider this an object of the full
+         * size, even if we only allocate the initial part we need. */
+        s = expand_to_usable(s, sizeof(sd_event_source));
 
         /* Note: we cannot use compound initialization here, because sizeof(sd_event_source) is likely larger
          * than what we allocated here. */