[xen staging] tools/tests/x86_emulator: avoid passing NULL to memcpy

[patchbot@xxxxxxx]

commit bdd49cc2f61510797a47ad81486be653633ab3ee
Author:     Edwin Török <edwin.torok@xxxxxxxxxx>
AuthorDate: Wed Mar 4 08:26:11 2026 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Mar 4 08:26:11 2026 +0100

    tools/tests/x86_emulator: avoid passing NULL to memcpy
    
    Fixes this `-fsanitize=undefined` error:
    ```
    test_x86_emulator.c:614:12: runtime error: null pointer passed as argument 
1, which is declared to never be null
    /usr/include/string.h:44:28: note: nonnull attribute specified here
    SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior 
test_x86_emulator.c:614:12
    ```
    
    Although this is more of a grey area: I don't see anything in the
    standard that'd forbid calling `memcpy` with NULL and 0, but `glibc`
    does specify it as non-null, which allows the compiler to make
    optimizations assuming it never is NULL, so this is undefined behaviour
    only on glibc.
    Best to avoid the potential undefined behaviour though.
    
    Signed-off-by: Edwin Török <edwin.torok@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 tools/tests/x86_emulator/test_x86_emulator.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/tests/x86_emulator/test_x86_emulator.c 
b/tools/tests/x86_emulator/test_x86_emulator.c
index f30568b7f1..20929b80c6 100644
--- a/tools/tests/x86_emulator/test_x86_emulator.c
+++ b/tools/tests/x86_emulator/test_x86_emulator.c
@@ -611,7 +611,8 @@ static int fetch(
     if ( verbose )
         printf("** %s(CS:%p,, %u,)\n", __func__, (void *)offset, bytes);
 
-    memcpy(p_data, (void *)offset, bytes);
+    if ( bytes )
+        memcpy(p_data, (void *)offset, bytes);
     return X86EMUL_OKAY;
 }
 
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