Fix crash when a mixin missing a target has an inner class

Doesn't crash-crash, but will refuse to apply the mixin on any non-missing targets

src/main/java/org/spongepowered/asm/mixin/transformer/MixinInfo.java

@@ -1246,6 +1246,13 @@ Set<String> getSyntheticInnerClasses() {
         return Collections.<String>unmodifiableSet(this.getState().getSyntheticInnerClasses());
     }
 
+    /**
+     * True if there are any user-defined inner classes for this mixin
+     */
+    boolean hasInnerClasses() {
+        return !this.getState().getInnerClasses().isEmpty();
+    }
+
     /**
      * Get the user-defined inner classes for this mixin
      */

src/main/java/org/spongepowered/asm/mixin/transformer/MixinPreProcessorStandard.java

@@ -201,11 +201,12 @@ final MixinPreProcessorStandard prepare(Extensions extensions) {
     }
 
     protected void prepareInnerClasses(Extensions extensions) {
-        InnerClassGenerator icg = extensions.<InnerClassGenerator>getGenerator(InnerClassGenerator.class);
-        for (String targetClassName : this.mixin.getDeclaredTargetClasses()) {
-            ClassInfo targetClassInfo = ClassInfo.forName(targetClassName);
-            for (String innerClass : this.mixin.getInnerClasses()) {
-                icg.registerInnerClass(this.mixin, targetClassInfo, innerClass);
+        if (this.mixin.hasInnerClasses()) {
+            InnerClassGenerator icg = extensions.<InnerClassGenerator>getGenerator(InnerClassGenerator.class);
+            for (ClassInfo targetClassInfo : this.mixin.getTargets()) {
+                for (String innerClass : this.mixin.getInnerClasses()) {
+                    icg.registerInnerClass(this.mixin, targetClassInfo, innerClass);
+                }
             }
         }
     }