[gdb/symtab] Handle setting line bp without debug line info

[gdb/symtab] Handle setting line bp without debug line info

When setting a breakpoint on a line in an executable without debug line info,
we run into an abort.

The problem occurs when calling set_default_source_symtab_and_line, which
calls select_source_symtab (0), which is where we try to find the line number
for main:
...
  /* Make the default place to list be the function `main'
     if one exists.  */
  block_symbol bsym = lookup_symbol (main_name (), 0, VAR_DOMAIN, 0);
  if (bsym.symbol != nullptr && SYMBOL_CLASS (bsym.symbol) == LOC_BLOCK)
    {
      symtab_and_line sal = find_function_start_sal (bsym.symbol, true);
      loc->set (sal.symtab, std::max (sal.line - (lines_to_list - 1), 1));
      return;
    }
...

However, due to the missing debug line info, find_function_start_sal returns a
sal with sal.symtab == 0:
...
(gdb) p /x sal
$2 = {pspace = 0x1a4a7f0, symtab = 0x0, symbol = 0x1d9e480, section = 0x1d5b398,
  msymbol = 0x0, line = 0x0, pc = 0x4004ab, end = 0x0, explicit_pc = 0x0,
  explicit_line = 0x0, is_stmt = 0x0, prob = 0x0, objfile = 0x0}
...
which eventually causes an segfault in create_sals_line_offset because
self->default_symtab->filename is accessed while self->default_symtab == NULL.

Fix this by handling sal.symtab == NULL in select_source_symtab.

Tested on x86_64-linux.

gdb/ChangeLog:

2020-10-17  Tom de Vries  <tdevries@suse.de>

	PR symtab/26317
	* source.c (select_source_symtab): Handling sal.symtab == NULL for
	symbol main.

gdb/testsuite/ChangeLog:

2020-10-17  Tom de Vries  <tdevries@suse.de>

	PR symtab/26317
	* gdb.dwarf2/dw2-main-no-line-number.exp: New file.