Remove usage of snprintf() from patty_ax25_ntop()

Remove usage of snprintf() from patty_ax25_ntop() to gain better control
over the behavior of how the 'len' argument and nul terminator are
handled

src/ax25.c

@@ -1,4 +1,3 @@
-#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
@@ -84,6 +83,21 @@ error_invalid_callsign:
     return -1;
 }
 
+static inline int expt(int base, int e) {
+    int ret = base,
+        i;
+
+    if (e == 0) {
+        return 1;
+    }
+
+    for (i=1; i<e; i++) {
+        ret *= base;
+    }
+
+    return ret;
+}
+
 int patty_ax25_ntop(const patty_ax25_addr *addr,
                     char *dest,
                     size_t len) {
@@ -96,7 +110,7 @@ int patty_ax25_ntop(const patty_ax25_addr *addr,
         uint8_t c;
 
         if (o == len) {
-            goto error_overflow;
+            goto error;
         }
 
         c = (addr->callsign[i] & 0xfe) >> 1;
@@ -109,25 +123,32 @@ int patty_ax25_ntop(const patty_ax25_addr *addr,
     }
 
     if (ssid) {
-        int formatted;
+        int digits = ssid > 9? 2: 1,
+            d;
 
-        if ((formatted = snprintf(&dest[o], 1 + len - o, "-%d", ssid)) < 0) {
-            goto error_format_ssid;
-        } else {
-            o += formatted;
+        if (o + 1 + digits > len) {
+            goto error;
+        }
+
+        dest[o++] = '-';
+
+        for (d=0; d<digits; d++) {
+            int place = expt(10, digits - d - 1),
+                num   = ((int)ssid % (place * 10)) / place;
+
+            dest[o++] = num + '0';
         }
     }
 
     if (o > len) {
-        goto error_overflow;
+        goto error;
     }
 
     dest[o] = '\0';
 
     return 0;
 
-error_format_ssid:
-error_overflow:
+error:
     errno = EOVERFLOW;
 
     return -1;