Fix redundant cppcheck warnings

https://github.com/Hamlib/Hamlib/issues/1351

src/amplifier.c

@@ -466,7 +466,7 @@ int HAMLIB_API amp_close(AMP *amp)
 
     amp_debug(RIG_DEBUG_VERBOSE, "%s called\n", __func__);
 
-    if (!amp || !amp->caps)
+    if (amp == NULL || amp->caps == NULL)
     {
         amp_debug(RIG_DEBUG_ERR, "%s: NULL ptr? amp=%p, amp->caps=%p\n", __func__, amp,
                   amp->caps);

src/iofunc.c

@@ -1307,7 +1307,7 @@ static int read_string_generic(hamlib_port_t *p,
     int i = 0;
     static int minlen = 1; // dynamic minimum length of rig response data
 
-    if (!p->asyncio && !direct)
+    if (p != NULL && !p->asyncio && !direct)
     {
         return -RIG_EINTERNAL;
     }
@@ -1461,7 +1461,7 @@ static int read_string_generic(hamlib_port_t *p,
 
     if (total_count > 1 && rxbuffer[0] == ';')
     {
-        while (rxbuffer[0] == ';' && rxbuffer[0] != 0 && total_count >  1)
+        while (rxbuffer[0] == ';' && total_count >  1)
         {
             memmove(rxbuffer, &rxbuffer[1], strlen((char *)rxbuffer) - 1);
             --total_count;