rework FindUnusedPID after heads up from Coverty

Avoid the special purpose Null PID while here.
Its still not perfect, but should be good enough for its intended use
when recording radio.

mythtv/libs/libmythtv/mpeg/mpegtables.cpp

@@ -717,24 +717,27 @@ uint ProgramMapTable::FindPIDs(uint           type,
 uint ProgramMapTable::FindUnusedPID(uint desired_pid)
 {
     uint pid = desired_pid;
-    while (FindPID(pid) >= 0)
+    if (pid >= MPEG_NULL_PID)
+        pid = 0x20;
+
+    while (FindPID(pid) != -1)
         pid += 0x10;
 
-    if (desired_pid <= 0x1fff)
+    if (pid < MPEG_NULL_PID)
         return pid;
 
     pid = desired_pid;
-    while (FindPID(desired_pid) >= 0)
-        desired_pid += 1;
+    while (FindPID(pid) != -1)
+        pid += 1;
 
-    if (desired_pid <= 0x1fff)
+    if (pid < MPEG_NULL_PID)
         return pid;
 
     pid = 0x20;
-    while (FindPID(desired_pid) >= 0)
-        desired_pid += 1;
+    while (FindPID(pid) != -1)
+        pid += 1;
 
-    return desired_pid & 0x1fff;
+    return pid & 0x1fff;
 }
 
 QString PSIPTable::toString(void) const

mythtv/libs/libmythtv/mpeg/mpegtables.h

@@ -225,6 +225,10 @@ enum
     FREESAT_SI_PID     = 0x0f01,
     FREESAT_EIT_PID    = 0x0f02,
     FREESAT_ST_EIT_PID = 0x0f03,
+
+    /// The all-ones PID value 0x1FFF indicates a Null TS Packet
+    /// introduced to maintain a constant bit rate of a TS Multiplex.
+    MPEG_NULL_PID      = 0x1fff,
 };
 
 /** \class TableID