[PATCH] system/physmem: Silence warning from ubsan

[Thomas Huth <thuth@redhat.com>]

From: Thomas Huth <thuth@redhat.com>

When compiling QEMU with --enable-ubsan there is a undefined behavior
warning when running the bios-tables-test for example:

 .../system/physmem.c:3243:13: runtime error: applying non-zero offset 262144 to null pointer
    #0 0x55ac1df5fbc4 in address_space_write_rom_internal .../system/physmem.c:3243:13

The problem is that buf is indeed NULL if the function is e.g. called
with type == FLUSH_CACHE. Add a check to fix the issue.

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 system/physmem.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/system/physmem.c b/system/physmem.c
index 130c148ffb5..00333ffa7f7 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -3240,8 +3240,10 @@ static inline MemTxResult address_space_write_rom_internal(AddressSpace *as,
             }
         }
         len -= l;
-        buf += l;
         addr += l;
+        if (buf) {
+            buf += l;
+        }
     }
     return MEMTX_OK;
 }
-- 
2.50.1