[SSK] Avoid crashes when signed prekey lookup fails.

// FREEBIE

Podfile

@@ -5,7 +5,7 @@ target 'Signal' do
     pod 'SocketRocket',               :git => 'https://github.com/facebook/SocketRocket.git'
     pod 'AxolotlKit',                 git: 'https://github.com/WhisperSystems/SignalProtocolKit.git'
     #pod 'AxolotlKit',                 path: '../SignalProtocolKit'
-    pod 'SignalServiceKit',           git: 'https://github.com/WhisperSystems/SignalServiceKit.git'
+    pod 'SignalServiceKit',           git: 'https://github.com/WhisperSystems/SignalServiceKit.git', branch: 'release/2.7.1'
     #pod 'SignalServiceKit',           path: '../SignalServiceKit'
     pod 'OpenSSL'
     pod 'PastelogKit',                '~> 1.3'

Podfile.lock

@@ -123,7 +123,7 @@ DEPENDENCIES:
   - PastelogKit (~> 1.3)
   - PureLayout
   - SCWaveformView (~> 1.0)
-  - SignalServiceKit (from `https://github.com/WhisperSystems/SignalServiceKit.git`)
+  - SignalServiceKit (from `https://github.com/WhisperSystems/SignalServiceKit.git`, branch `release/2.7.1`)
   - SocketRocket (from `https://github.com/facebook/SocketRocket.git`)
   - ZXingObjC
 
@@ -131,6 +131,7 @@ EXTERNAL SOURCES:
   AxolotlKit:
     :git: https://github.com/WhisperSystems/SignalProtocolKit.git
   SignalServiceKit:
+    :branch: release/2.7.1
     :git: https://github.com/WhisperSystems/SignalServiceKit.git
   SocketRocket:
     :git: https://github.com/facebook/SocketRocket.git
@@ -140,7 +141,7 @@ CHECKOUT OPTIONS:
     :commit: 945c04f9e70f7119d2ada4edb938849f3b7febab
     :git: https://github.com/WhisperSystems/SignalProtocolKit.git
   SignalServiceKit:
-    :commit: 93219e4d23e07b3734fb8df4a2d244b26fcecd59
+    :commit: 54736426f99ed4bc010522ff07aeab45f7cd2ea2
     :git: https://github.com/WhisperSystems/SignalServiceKit.git
   SocketRocket:
     :commit: 41b57bb2fc292a814f758441a05243eb38457027
@@ -172,6 +173,6 @@ SPEC CHECKSUMS:
   YapDatabase: b1e43555a34a5298e23a045be96817a5ef0da58f
   ZXingObjC: bf15b3814f7a105b6d99f47da2333c93a063650a
 
-PODFILE CHECKSUM: 48dbf2fb380d626bb799a782dd41b6bf1e466506
+PODFILE CHECKSUM: cdc752b941beadf2808273dea7c78e4f6a9a0ba6
 
 COCOAPODS: 1.1.1

Signal.xcworkspace/xcshareddata/Signal.xcscmblueprint

@@ -24,6 +24,11 @@
   "DVTSourceControlWorkspaceBlueprintVersion" : 204,
   "DVTSourceControlWorkspaceBlueprintRelativePathToProjectKey" : "Signal.xcworkspace",
   "DVTSourceControlWorkspaceBlueprintRemoteRepositoriesKey" : [
+    {
+      "DVTSourceControlWorkspaceBlueprintRemoteRepositoryURLKey" : "github.com:WhisperSystems\/SignalProtocolKit.git",
+      "DVTSourceControlWorkspaceBlueprintRemoteRepositorySystemKey" : "com.apple.dt.Xcode.sourcecontrol.Git",
+      "DVTSourceControlWorkspaceBlueprintRemoteRepositoryIdentifierKey" : "01DE8628B025BC69C8C7D8B4612D57BE2C08B62C"
+    },
     {
       "DVTSourceControlWorkspaceBlueprintRemoteRepositoryURLKey" : "github.com:FredericJacobs\/TextSecureKit.git",
       "DVTSourceControlWorkspaceBlueprintRemoteRepositorySystemKey" : "com.apple.dt.Xcode.sourcecontrol.Git",