gdb: Guard against NULL dereference in dwarf2_init_integer_type

gdb: Guard against NULL dereference in dwarf2_init_integer_type

In this commit:

    commit eb77c9df9f6d2f7aa644a170280fe31ce080f887
    Date:   Thu Oct 18 14:04:27 2018 +0100

        gdb: Handle ICC's unexpected void return type

A potential dereference of a NULL pointer was introduced if a
DW_TAG_base_type is missing a DW_AT_name attribute.

I have taken this opportunity to fix a slight confusion that existed
in the test also added in the above commit, the test had two C
variables, declared like this:

    int var_a = 5;

    void *var_ptr = &var_a;

However, the fake DWARF in the test script declared them like this:

    void var_a = 5;

    void *var_ptr = &var_a;

This wasn't a problem as the test never uses 'var_a' directly, this
only exists so 'var_ptr' can be initialised.  However, it seemed worth
fixing.

I've also added a test for a DW_TAG_base_type with a missing
DW_AT_name, as clearly there's not test currently that covers this
(the original patch tested cleanly).  I can confirm that the new test
causes GDB to crash before this patch, and passes with this patch.

gdb/ChangeLog:

	* dwarf2read.c (dwarf2_init_integer_type): Check for name being
	NULL before dereferencing it.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/void-type.exp: Rename types, and make var_a an 'int'.
	* gdb.dwarf2/missing-type-name.exp: New file.