py/qstr: Raise exception in qstr_from_strn if str to intern is too long.

The string length being longer than the allowed qstr length can happen in
many locations, for example in the parser with very long variable names.
Without an explicit check that the length is within range (as done in this
patch) the code would exhibit crashes and strange behaviour with truncated
strings.

py/qstr.c

@@ -31,6 +31,7 @@
 #include "py/mpstate.h"
 #include "py/qstr.h"
 #include "py/gc.h"
+#include "py/runtime.h"
 
 // NOTE: we are using linear arrays to store and search for qstr's (unique strings, interned strings)
 // ultimately we will replace this with a static hash table of some kind
@@ -192,12 +193,17 @@ qstr qstr_from_str(const char *str) {
 }
 
 qstr qstr_from_strn(const char *str, size_t len) {
-    assert(len < (1 << (8 * MICROPY_QSTR_BYTES_IN_LEN)));
     QSTR_ENTER();
     qstr q = qstr_find_strn(str, len);
     if (q == 0) {
         // qstr does not exist in interned pool so need to add it
 
+        // check that len is not too big
+        if (len >= (1 << (8 * MICROPY_QSTR_BYTES_IN_LEN))) {
+            QSTR_EXIT();
+            mp_raise_msg(&mp_type_RuntimeError, "name too long");
+        }
+
         // compute number of bytes needed to intern this string
         size_t n_bytes = MICROPY_QSTR_BYTES_IN_HASH + MICROPY_QSTR_BYTES_IN_LEN + len + 1;