py/compile: Give the compiler a hint about num nodes being non-zero.

Without this, newer versions of gcc (eg 11.2.0) used with -O2 can warn
about `q_ptr` being maybe uninitialized, because it doesn't know that there
is at least one qstr being written in to this (alloca'd) memory.

As part of this, change the type of `n` to `size_t` so the compiler knows
it's unsigned and can generate better code.

Code size change for this commit:

       bare-arm:   -28 -0.049%
    minimal x86:    -4 -0.002%
       unix x64:    +0 +0.000%
    unix nanbox:   -16 -0.003%
          stm32:   -24 -0.006% PYBV10
         cc3200:   -32 -0.017%
        esp8266:    +8 +0.001% GENERIC
          esp32:   -52 -0.003% GENERIC
            nrf:   -24 -0.013% pca10040
            rp2:   -32 -0.006% PICO
           samd:   -28 -0.020% ADAFRUIT_ITSYBITSY_M4_EXPRESS

Signed-off-by: Damien George <damien@micropython.org>

py/compile.c

@@ -1115,14 +1115,19 @@ STATIC void do_import_name(compiler_t *comp, mp_parse_node_t pn, qstr *q_base) {
             if (!is_as) {
                 *q_base = MP_PARSE_NODE_LEAF_ARG(pns->nodes[0]);
             }
-            int n = MP_PARSE_NODE_STRUCT_NUM_NODES(pns);
-            int len = n - 1;
-            for (int i = 0; i < n; i++) {
+            size_t n = MP_PARSE_NODE_STRUCT_NUM_NODES(pns);
+            if (n == 0) {
+                // There must be at least one node in this PN_dotted_name.
+                // Let the compiler know this so it doesn't warn, and can generate better code.
+                MP_UNREACHABLE;
+            }
+            size_t len = n - 1;
+            for (size_t i = 0; i < n; i++) {
                 len += qstr_len(MP_PARSE_NODE_LEAF_ARG(pns->nodes[i]));
             }
             char *q_ptr = mp_local_alloc(len);
             char *str_dest = q_ptr;
-            for (int i = 0; i < n; i++) {
+            for (size_t i = 0; i < n; i++) {
                 if (i > 0) {
                     *str_dest++ = '.';
                 }
@@ -1135,7 +1140,7 @@ STATIC void do_import_name(compiler_t *comp, mp_parse_node_t pn, qstr *q_base) {
             mp_local_free(q_ptr);
             EMIT_ARG(import, q_full, MP_EMIT_IMPORT_NAME);
             if (is_as) {
-                for (int i = 1; i < n; i++) {
+                for (size_t i = 1; i < n; i++) {
                     EMIT_ARG(attr, MP_PARSE_NODE_LEAF_ARG(pns->nodes[i]), MP_EMIT_ATTR_LOAD);
                 }
             }