repl.c: Don't read past the end of import_str.

asan considers that memcmp(p, q, N) is permitted to access N bytes at
each of p and q, even for values of p and q that have a difference
earlier.  Accessing additional values is frequently done in practice,
reading 4 or more bytes from each input at a time for efficiency, so
when completing "non_exist<TAB>" in the repl, this causes a diagnostic:

```
==16938==ERROR: AddressSanitizer: global-buffer-overflow on address 0x555555cd8dc8 at pc 0x7ffff726457b bp 0x7fffffffda20 sp 0x7fffffffd1d0
READ of size 9 at 0x555555cd8dc8 thread T0
    #0 0x7ffff726457a  (/usr/lib/x86_64-linux-gnu/libasan.so.5+0xb857a)
    #1 0x555555b0e82a in mp_repl_autocomplete ../../py/repl.c:301
    #2 0x555555c89585 in readline_process_char ../../lib/mp-readline/readline.c:225
    #3 0x555555c8ac6e in readline ../../lib/mp-readline/readline.c:513
    #4 0x555555b8dcbd in do_repl /home/jepler/src/micropython/ports/unix/main.c:194
    adafruit#5 0x555555b90859 in main_ /home/jepler/src/micropython/ports/unix/main.c:673
    adafruit#6 0x555555b90a3a in main /home/jepler/src/micropython/ports/unix/main.c:436
    adafruit#7 0x7ffff619a09a in __libc_start_main ../csu/libc-start.c:308
    adafruit#8 0x55555595fd69 in _start (/home/jepler/src/micropython/ports/unix/micropython-coverage+0x40bd69)

0x555555cd8dc8 is located 0 bytes to the right of global variable 'import_str' defined in '../../py/repl.c:285:23' (0x555555cd8dc0) of size 8
  'import_str' is ascii string 'import '
SUMMARY: AddressSanitizer: global-buffer-overflow (/usr/lib/x86_64-linux-gnu/libasan.so.5+0xb857a)
Shadow bytes around the buggy address:
  0x0aab2ab93160: 04 f9 f9 f9 f9 f9 f9 f9 04 f9 f9 f9 f9 f9 f9 f9
  0x0aab2ab93170: 05 f9 f9 f9 f9 f9 f9 f9 04 f9 f9 f9 f9 f9 f9 f9
  0x0aab2ab93180: 06 f9 f9 f9 f9 f9 f9 f9 06 f9 f9 f9 f9 f9 f9 f9
  0x0aab2ab93190: 05 f9 f9 f9 f9 f9 f9 f9 00 00 f9 f9 f9 f9 f9 f9
  0x0aab2ab931a0: 00 00 00 00 00 00 00 00 04 f9 f9 f9 f9 f9 f9 f9
=>0x0aab2ab931b0: 00 00 00 00 00 00 00 00 00[f9]f9 f9 f9 f9 f9 f9
  0x0aab2ab931c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aab2ab931d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0aab2ab931e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 04 f9
  0x0aab2ab931f0: f9 f9 f9 f9 00 00 00 00 00 00 00 00 00 f9 f9 f9
  0x0aab2ab93200: f9 f9 f9 f9 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==16938==ABORTING
```

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

Author: jepler

py/repl.c

@@ -297,7 +297,7 @@ size_t mp_repl_autocomplete(const char *str, size_t len, const mp_print_t *print
     if (q_first == 0) {
         // If there're no better alternatives, and if it's first word
         // in the line, try to complete "import".
-        if (s_start == org_str && s_len > 0) {
+        if (s_start == org_str && s_len > 0 && s_len < 7) {
             if (memcmp(s_start, import_str, s_len) == 0) {
                 *compl_str = import_str + s_len;
                 return sizeof(import_str) - 1 - s_len;