gsm48_decode_bcd_number2(): fix output truncation

Thanks to the new unit test for BCD number encoding / decoding, it was
discovered that gsm48_decode_bcd_number2() does not properly handle
encoded LV if the output buffer size is equal to the original MSISDN
length + 1 (\0-terminator): one digit is lost.

For example, decoding of 15-digit long MSISDN to a buffer of size
16 (15 digits + 1 for \0) would give us only 14 digits.

The problem was that 'output_len' was being decremented before
checking the remaining buffer length and writing a digit to it.
As a result, the maximum length was always one byte shorter.

Change-Id: I61d49387fedbf7b238e21540a5eff22f6861e27a
Fixes: OS#4025

src/gsm/gsm48_ie.c

@@ -88,16 +88,16 @@ int gsm48_decode_bcd_number2(char *output, size_t output_len,
 
 	for (i = 1 + h_len; i <= in_len; i++) {
 		/* lower nibble */
-		output_len--;
 		if (output_len <= 1)
 			break;
 		*output++ = bcd_num_digits[bcd_lv[i] & 0xf];
+		output_len--;
 
 		/* higher nibble */
-		output_len--;
 		if (output_len <= 1)
 			break;
 		*output++ = bcd_num_digits[bcd_lv[i] >> 4];
+		output_len--;
 	}
 	if (output_len >= 1)
 		*output++ = '\0';

tests/gsm0408/gsm0408_test.ok

@@ -167,14 +167,14 @@ BSD number encoding / decoding test
     -   Actual: (rc=9) '0821436587092143f5'
   - Decoding HEX (buffer limit=16) '0821436587092143f5'...
     - Expected: (rc=0) '123456789012345'
-    -   Actual: (rc=0) '12345678901234'
+    -   Actual: (rc=0) '123456789012345'
 - Running test: to be truncated 20-digit MSISDN
   - Encoding ASCII (buffer limit=9) '12345678901234567890'...
     - Expected: (rc=-5) ''
     -   Actual: (rc=-5) ''
   - Decoding HEX (buffer limit=16) '0a21436587092143658709'...
     - Expected: (rc=0) '123456789012345'
-    -   Actual: (rc=0) '12345678901234'
+    -   Actual: (rc=0) '123456789012345'
 - Running test: LV incorrect length
   - Decoding HEX (buffer limit=0) '05214365'...
     - Expected: (rc=-5) '(none)'