[PATCH v2 2/7] [gdb/tui] Fix underflow in tui_find_backward_disassembly_start_address

[Tom de Vries <tdevries@suse.de>]

In the tui-disasm.c selftests, we do:
...
      SELF_CHECK (tui_find_disassembly_address (gdbarch, 0, -1) == 0);
...

This causes tui_find_backward_disassembly_start_address to be called with
addr == 0, causing underflow in:
...
  bound_minimal_symbol msym
    = lookup_minimal_symbol_by_pc_section (addr - 1, nullptr,
                                           lookup_msym_prefer::TEXT,
                                           &msym_prev);
...

Fix this by handling addr == 0 in tui_find_backward_disassembly_start_address.
---
 gdb/tui/tui-disasm.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/gdb/tui/tui-disasm.c b/gdb/tui/tui-disasm.c
index 923d4373e8f..7dcb59e6264 100644
--- a/gdb/tui/tui-disasm.c
+++ b/gdb/tui/tui-disasm.c
@@ -161,6 +161,12 @@ tui_disassemble (struct gdbarch *gdbarch,
 static CORE_ADDR
 tui_find_backward_disassembly_start_address (CORE_ADDR addr)
 {
+  if (addr == 0)
+    {
+      /* We cannot go backwards from zero.  */
+      return addr;
+    }
+
   bound_minimal_symbol msym_prev;
   bound_minimal_symbol msym
     = lookup_minimal_symbol_by_pc_section (addr - 1, nullptr,
-- 
2.51.0