Adds precondition for checking if user is not merging on itself

Author: flovilmart

Parse/Parse/Internal/PFAssert.h

@@ -75,6 +75,11 @@ return rval;\
 #define PFBailTaskIfError(condition, error) \
 PFBailIfError(condition, error, [BFTask taskWithError:error])
 
+#define PFPrecondition(condition, description, ...) \
+if (!(condition)) { \
+return [PFErrorUtilities errorWithCode:-1 message:[NSString stringWithFormat:description, ##__VA_ARGS__]];\
+}
+
 /**
  Raises an `NSInternalInconsistencyException`. Use `description` to supply the way to fix the exception.
  */

Parse/Parse/PFUser.m

@@ -509,7 +509,7 @@ - (BFTask *)signUpAsync:(BFTask *)toAwait {
                 // self doesn't have any outstanding saves, so we can safely merge its operations
                 // into the current user.
 
-                PFConsistencyAssert(!self._current, @"Attempt to merge currentUser with itself.");
+                PFPrecondition(!self._current, @"Attempt to merge currentUser with itself.");
 
                 @synchronized ([currentUser lock]) {
                     NSString *oldUsername = [currentUser.username copy];