fix signed/unsigned comparison warning

lib/include/srslte/common/pdu.h

@@ -137,7 +137,7 @@ public:
         nof_subheaders++;
       }
 
-      if (ret && (ptr - init_ptr) >= pdu_len) {
+      if (ret && ((ptr - init_ptr) >= (int32_t)pdu_len)) {
         // stop processing last subheader indicates another one but all bytes are consume
         nof_subheaders = 0;
         INFO("Corrupted MAC PDU - all bytes have been consumed (pdu_len=%d)\n", pdu_len);
@@ -145,7 +145,7 @@ public:
           log->info_hex(init_ptr, pdu_len, "Corrupted MAC PDU - all bytes have been consumed (pdu_len=%d)\n", pdu_len);
         }
       }
-    } while (ret && (nof_subheaders + 1) < (int)max_subheaders && pdu_len > (ptr - init_ptr));
+    } while (ret && (nof_subheaders + 1) < (int)max_subheaders && ((int32_t)pdu_len > (ptr - init_ptr)));
 
     for (int i = 0; i < nof_subheaders; i++) {
       subheaders[i].read_payload(&ptr);