syscall: check inotify() and eventfd() return value

linux-user/syscall.c:555:25: warning: Out of bound memory access (accessed memory precedes memory block)
    target_fd_trans[fd] = trans;
    ~~~~~~~~~~~~~~~~~~~~^~~~~~~

Reported-by: Clang Static Analyzer
Suggested-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

linux-user/syscall.c

@@ -11742,7 +11742,9 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
 #if defined(TARGET_NR_inotify_init) && defined(__NR_inotify_init)
     case TARGET_NR_inotify_init:
         ret = get_errno(sys_inotify_init());
-        fd_trans_register(ret, &target_inotify_trans);
+        if (ret >= 0) {
+            fd_trans_register(ret, &target_inotify_trans);
+        }
         break;
 #endif
 #ifdef CONFIG_INOTIFY1
@@ -11750,7 +11752,9 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
     case TARGET_NR_inotify_init1:
         ret = get_errno(sys_inotify_init1(target_to_host_bitmask(arg1,
                                           fcntl_flags_tbl)));
-        fd_trans_register(ret, &target_inotify_trans);
+        if (ret >= 0) {
+            fd_trans_register(ret, &target_inotify_trans);
+        }
         break;
 #endif
 #endif
@@ -11916,7 +11920,9 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
 #if defined(TARGET_NR_eventfd)
     case TARGET_NR_eventfd:
         ret = get_errno(eventfd(arg1, 0));
-        fd_trans_register(ret, &target_eventfd_trans);
+        if (ret >= 0) {
+            fd_trans_register(ret, &target_eventfd_trans);
+        }
         break;
 #endif
 #if defined(TARGET_NR_eventfd2)
@@ -11930,7 +11936,9 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
             host_flags |= O_CLOEXEC;
         }
         ret = get_errno(eventfd(arg1, host_flags));
-        fd_trans_register(ret, &target_eventfd_trans);
+        if (ret >= 0) {
+            fd_trans_register(ret, &target_eventfd_trans);
+        }
         break;
     }
 #endif