[PATCH v2 4/4] tools/tests/x86_emulator: avoid passing NULL to memcpy

[Edwin Török <edwin.torok@xxxxxxxxxx>]

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 `memset` 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>
---
 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 3a03ea0352..87c1289afa 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;
 }
 
-- 
2.47.3