OLMKit: More zeroing

xcode/OLMKit/OLMInboundGroupSession.m

@@ -143,6 +143,7 @@
     }
     plaintextData.length = plaintextLength;
     NSString *plaintext = [[NSString alloc] initWithData:plaintextData encoding:NSUTF8StringEncoding];
+    [plaintextData resetBytesInRange:NSMakeRange(0, plaintextData.length)];
 
     if (messageIndex)
     {

xcode/OLMKit/OLMSession.m

@@ -287,6 +287,7 @@
     }
     plaintextData.length = plaintextLength;
     NSString *plaintext = [[NSString alloc] initWithData:plaintextData encoding:NSUTF8StringEncoding];
+    [plaintextData resetBytesInRange:NSMakeRange(0, plaintextData.length)];
     return plaintext;
 }