qga: Fix ubsan warning

When compiling QEMU with --enable-ubsan there is a undefined behavior
warning when running "make check":

 .../qga/commands-linux.c:452:15: runtime error: applying non-zero offset 5 to null pointer
 #0 0x55ea7b89450c in build_guest_fsinfo_for_pci_dev ..../qga/commands-linux.c:452:15

Fix it by avoiding the additional pointer variable here and use an
"offset" integer variable instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Kostiantyn Kostiuk <kkostiuk@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250730072709.27077-1-thuth@redhat.com
Signed-off-by: Kostiantyn Kostiuk <kkostiuk@redhat.com>

qga/commands-linux.c

@@ -400,10 +400,10 @@ static bool build_guest_fsinfo_for_pci_dev(char const *syspath,
                                            Error **errp)
 {
     unsigned int pci[4], host, hosts[8], tgt[3];
-    int i, nhosts = 0, pcilen;
+    int i, offset, nhosts = 0, pcilen;
     GuestPCIAddress *pciaddr = disk->pci_controller;
     bool has_ata = false, has_host = false, has_tgt = false;
-    char *p, *q, *driver = NULL;
+    char *p, *driver = NULL;
     bool ret = false;
 
     p = strstr(syspath, "/devices/pci");
@@ -445,13 +445,13 @@ static bool build_guest_fsinfo_for_pci_dev(char const *syspath,
 
     p = strstr(syspath, "/ata");
     if (p) {
-        q = p + 4;
+        offset = 4;
         has_ata = true;
     } else {
         p = strstr(syspath, "/host");
-        q = p + 5;
+        offset = 5;
     }
-    if (p && sscanf(q, "%u", &host) == 1) {
+    if (p && sscanf(p + offset, "%u", &host) == 1) {
         has_host = true;
         nhosts = build_hosts(syspath, p, has_ata, hosts,
                              ARRAY_SIZE(hosts), errp);