gsm: make osmo_imsi_str_valid() NULL-safe

No callers that would pass NULL exist, but let's check against NULL from the
start.

Fixup for recent change I1e94f5b0717b947d2a7a7d36bacdf04a75cb3522.

Change-Id: I111fbf29228929f2cd6ffa06bcb1f69da223224e

src/gsm/gsm23003.c

@@ -33,6 +33,8 @@ static bool is_n_digits(const char *str, int min_digits, int max_digits)
 	/* Use unsigned char * to avoid a compiler warning of
 	 * "error: array subscript has type 'char' [-Werror=char-subscripts]" */
 	const unsigned char *pos = (const unsigned char *)str;
+	if (!pos)
+		return min_digits < 1;
 	for (len = 0; *pos && len < max_digits; len++, pos++)
 		if (!isdigit(*pos))
 			return false;

tests/gsm23003/gsm23003_test.c

@@ -50,6 +50,7 @@ static struct {
 	{ "1234567\n123456", false },
 	{ "123456\t123456", false },
 	{ "123456\r123456", false },
+	{ NULL, false },
 };
 
 bool test_valid_imsi()
@@ -92,6 +93,7 @@ static struct {
 	{ "1234567\n123456", false },
 	{ "123456\t123456", false },
 	{ "123456\r123456", false },
+	{ NULL, false },
 };
 
 bool test_valid_msisdn()

tests/gsm23003/gsm23003_test.ok

@@ -19,6 +19,7 @@
 123456'
 17: expect=false result=false imsi='123456	123456'
 18: expect=false result=false imsi='123456
123456'
+19: expect=false result=false imsi='(null)'
 ----- test_valid_msisdn
  0: expect=false result=false msisdn=''
  1: expect=false result=false msisdn=' '
@@ -40,3 +41,4 @@
 123456'
 17: expect=false result=false msisdn='123456	123456'
 18: expect=false result=false msisdn='123456
123456'
+19: expect=false result=false msisdn='(null)'