Fix internal error on DW_OP_bregx(-1)

Fix internal error on DW_OP_bregx(-1)

https://bugzilla.redhat.com/show_bug.cgi?id=1270564#c15
https://bugzilla.redhat.com/attachment.cgi?id=1081772

clang-3.5.0-9.fc22.x86_64
 <3><22b2>: Abbrev Number: 69 (DW_TAG_variable)
    <22b3>   DW_AT_location    : 7 byte block: 92 ff ff ff ff f 0	(DW_OP_bregx: 4294967295 (r-1) 0)
    <22bb>   DW_AT_name        : (indirect string, offset: 0x2a36): texture_data
    <22c1>   DW_AT_type        : <0x1d3>

(gdb) p variable
warning: Unmapped DWARF Register #-1 encountered.
regcache.c:177: internal-error: register_size: Assertion `regnum >= 0 && regnum < (gdbarch_num_regs (gdbarch) + gdbarch_num_pseudo_regs
(gdbarch))' failed.
[...]
Quit this debugging session? (y or n) FAIL: gdb.dwarf2/dw2-regno-invalid.exp: p variable (GDB internal error)

-> (x86_64)
(gdb) p variable
warning: Unmapped DWARF Register #-1 encountered.
Invalid register #-1, expecting 0 <= # < 220
(gdb) PASS: gdb.dwarf2/dw2-regno-invalid.exp: p variable
-> (i386)
(gdb) p variable
Invalid register #104, expecting 0 <= # < 104
(gdb) PASS: gdb.dwarf2/dw2-regno-invalid.exp: p variable

GDB calls gdbarch_dwarf2_reg_to_regnum() first which returns -1 in the x86_64
case
  if (regnum == -1)
    warning (_("Unmapped DWARF Register #%d encountered."), reg);
but in i386 case it does:
  /* This will hopefully provoke a warning.  */
  return gdbarch_num_regs (gdbarch) + gdbarch_num_pseudo_regs (gdbarch);
and the default implementation is a nop, leaving whatever register number
the DWARF specified.

gdb/ChangeLog
2015-10-20  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* findvar.c (address_from_register): Check REGNUM validity.

gdb/testsuite/ChangeLog
2015-10-20  Jan Kratochvil  <jan.kratochvil@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	* gdb.dwarf2/dw2-regno-invalid.exp: New file.
	* lib/dwarf.exp (Dwarf): Add DW_OP_bregx.