[gdb/build] Fix build with gcc 4.8.5

[gdb/build] Fix build with gcc 4.8.5

When building with gcc 4.8.5, I run into:
...
user-regs.c:85:1: error: could not convert \
  ‘{0l, (& builtin_user_regs.gdb_user_regs::first)}’ from \
  ‘<brace-enclosed initializer list>’ to ‘gdb_user_regs’
 };
 ^
...

Fix this by removing the initialization and handling regs.last == nullptr in
append_user_reg.

Tested on x86_64-linux.