Minor refactor of aglib/regs.py:get_register (#2583)

TL;DR: Don't check `frame` if it is not `None` already.

pwndbg/aglib/regs.py

@@ -41,11 +41,11 @@ def get_register(
 ) -> pwndbg.dbg_mod.Value | None:
     if frame is None:
         frame = pwndbg.dbg.selected_frame()
-    if frame is None:
-        # `read_reg` will return None when it catches this exception. This
-        # mirrors how a `gdb.error` causes `read_reg` to return `None` in
-        # gdblib when no frame is selected.
-        raise pwndbg.dbg_mod.Error("No currently selected frame to read registers from")
+        if frame is None:
+            # `read_reg` will return None when it catches this exception. This
+            # mirrors how a `gdb.error` causes `read_reg` to return `None` in
+            # gdblib when no frame is selected.
+            raise pwndbg.dbg_mod.Error("No currently selected frame to read registers from")
 
     regs = regs_in_frame(frame)