py/compile: Throw SyntaxError instead of asserting.

This condition corresponds to invalid asm code like
```
@micropython.asm_rv32
def l():
    a=di(a2, a2, -1)
```
and possibly other forms where nodes[0] is not a STRUCT.

Signed-off-by: Jeff Epler <jepler@gmail.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/py/compile.c b/py/compile.c
index 7a1151bcd..945ee2b2d 100644
--- a/py/compile.c
+++ b/py/compile.c
@@ -3277,7 +3277,9 @@ static void compile_scope_inline_asm(compiler_t *comp, scope_t *scope, pass_kind
         }
 
         // check structure of parse node
-        assert(MP_PARSE_NODE_IS_STRUCT(pns2->nodes[0]));
+        if (!MP_PARSE_NODE_IS_STRUCT(pns2->nodes[0])) {
+            goto not_an_instruction;
+        }
         if (!MP_PARSE_NODE_IS_NULL(pns2->nodes[1])) {
             goto not_an_instruction;
         }