Use `isNil` to check null pointers

Signed-off-by: Christopher Arndt <chris@chrisarndt.de>

examples/miditranspose.nim

@@ -26,7 +26,8 @@ proc instantiate(descriptor: ptr Lv2Descriptor; sampleRate: cdouble;
     let amp: ptr MidiTransposePlugin = createShared(MidiTransposePlugin)
     amp.map = cast[ptr UridMap](lv2FeaturesData(features, lv2UridMap))
 
-    if amp.map == nil:
+    if amp.map.isNil:
+        freeShared(amp)
         return nil
 
     amp.midi_urid = amp.map.map(amp.map.handle, lv2MidiMidiEvent)

src/nymph/atom/util.nim

@@ -31,7 +31,7 @@ proc atomTotalSize*(atom: ptr Atom): uint32 {.inline.} =
 ## Return true iff `atom` is null.
 ##
 proc atomIsNull*(atom: ptr Atom): bool {.inline.} =
-    return atom == nil or (atom.`type` == Urid(0) and atom.size == 0)
+    return atom.isNil or (atom.`type` == Urid(0) and atom.size == 0)
 
 ##
 ## Return true iff `a` is equal to `b`.

src/nymph/util.nim

@@ -7,11 +7,11 @@ import core
 ## present but have nil data.
 
 proc lv2FeaturesData*(features: ptr UncheckedArray[ptr Lv2Feature], uri: string): pointer =
-    if features != nil:
+    if not features.isNil:
         var i = 0
         while true:
             let feature = features[i]
-            if feature == nil:
+            if feature.isNil:
                 break
             
             if feature[].uri == uri.cstring: