[gdb/symtab] Remove superfluous end-of-sequence marker

While working on PR26935 I noticed that when running test-case
gdb.base/morestack.exp with target board unix/-m32/-fPIE/-pie and ld linker,
I get this linetable fragment for morestack.S using readelf -wL:
...
CU: ../../../../libgcc/config/i386/morestack.S:
Line number    Starting address    View    Stmt
109               0xc9c               x
  ...
838               0xe03               x
  -               0xe04

636                   0               x
637                 0x3               x
  -                 0x4
...
but with "maint info line-table" I get:
...
INDEX  LINE   ADDRESS            IS-STMT
0      END    0x00000004         Y
1      109    0x00000c9c         Y
  ...
110    838    0x00000e03         Y
111    END    0x00000e04         Y
...

So, apparently the entries with addresses 0x0 and 0x3 are filtered out
because the addresses are out of range, but the same doesn't happen with the
end-of-seq terminator.

Fix this by filtering out end-of-seq terminators that do not actually
terminate anything.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-01-04  Tom de Vries  <tdevries@suse.de>

	* buildsym.c (buildsym_compunit::record_line): Filter out end-of-seq
	terminators that do not terminate anything.

gdb/testsuite/ChangeLog:

2021-01-04  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dw2-out-of-range-end-of-seq.exp: New file.