Issue error on erroneous expression

A while back I discovered that this does not issue an error:

    (gdb) p $x = (void * ) 57
    $3 = (void *) 0x39
    (gdb) p $x + 7 = 3
    $6 = (void *) 0x3

This patch fixes the bug.
Regression tested on x86-64 Fedora 36.

Reviewed-By: Bruno Larsen <blarsen@redhat.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=19312

gdb/testsuite/gdb.base/gdbvars.exp

@@ -66,6 +66,14 @@ proc test_convenience_variables {} {
     gdb_test "print \$arr = {0, 1, 2, 3}" \
 	" = \\{0, 1, 2, 3\\}" \
 	"Set convenience variable to different array value"
+
+    gdb_test "print \$ptr = (void *) 7" \
+	" = \\(void \\*\\) 0x7" \
+	"set convenience variable to pointer value"
+    # This used to "succeed".
+    gdb_test "print \$ptr + 23 = 97" \
+	"Left operand of assignment is not an lvalue." \
+	"reject invalid assignment"
 }
 
 proc test_convenience_functions {} {

gdb/valarith.c

@@ -93,7 +93,7 @@ value_ptradd (struct value *arg1, LONGEST arg2)
 
   result = value_from_pointer (valptrtype,
 			       value_as_address (arg1) + sz * arg2);
-  if (result->lval () != lval_internalvar)
+  if (arg1->lval () != lval_internalvar)
     result->set_component_location (arg1);
   return result;
 }