py: Fix wrong assumption that m_renew will not move if shrinking

In both parse.c and qstr.c, an internal chunking allocator tidies up
by calling m_renew to shrink an allocated chunk to the size used, and
assumes that the chunk will not move.  However, when MICROPY_ENABLE_GC
is false, m_renew calls the system realloc, which does not guarantee
this behaviour.  Environments where realloc may return a different
pointer include:

(1) mbed-os with MBED_HEAP_STATS_ENABLED (which adds a wrapper around
malloc & friends; this is where I was hit by the bug);

(2) valgrind on linux (how I diagnosed it).

The fix is to call m_renew_maybe with allow_move=false.

py/parse.c

@@ -1010,9 +1010,10 @@ mp_parse_tree_t mp_parse(mp_lexer_t *lex, mp_parse_input_kind_t input_kind) {
 
     // truncate final chunk and link into chain of chunks
     if (parser.cur_chunk != NULL) {
-        (void)m_renew(byte, parser.cur_chunk,
+        (void)m_renew_maybe(byte, parser.cur_chunk,
             sizeof(mp_parse_chunk_t) + parser.cur_chunk->alloc,
-            sizeof(mp_parse_chunk_t) + parser.cur_chunk->union_.used);
+            sizeof(mp_parse_chunk_t) + parser.cur_chunk->union_.used,
+            false);
         parser.cur_chunk->alloc = parser.cur_chunk->union_.used;
         parser.cur_chunk->union_.next = parser.tree.chunk;
         parser.tree.chunk = parser.cur_chunk;

py/qstr.c

@@ -199,7 +199,7 @@ qstr qstr_from_strn(const char *str, size_t len) {
             byte *new_p = m_renew_maybe(byte, MP_STATE_VM(qstr_last_chunk), MP_STATE_VM(qstr_last_alloc), MP_STATE_VM(qstr_last_alloc) + n_bytes, false);
             if (new_p == NULL) {
                 // could not grow existing memory; shrink it to fit previous
-                (void)m_renew(byte, MP_STATE_VM(qstr_last_chunk), MP_STATE_VM(qstr_last_alloc), MP_STATE_VM(qstr_last_used));
+                (void)m_renew_maybe(byte, MP_STATE_VM(qstr_last_chunk), MP_STATE_VM(qstr_last_alloc), MP_STATE_VM(qstr_last_used), false);
                 MP_STATE_VM(qstr_last_chunk) = NULL;
             } else {
                 // could grow existing memory