[ObjC] Fix over-release in GPBTextFormatForMessage

When handling deeply nested unknown fields, the `subIndent` string was being over-released in `GPBTextFormatForMessage`, leading to crashes when generating text format for messages with deeply nested unknown fields.

The added test case reproduces this crash by creating a message with unknown fields nested deep enough to cause the indent string to be heap-allocated, and then adding siblings at that depth, triggering the incorrect release.

Removing the redundant `[subIndent release]` resolves the issue.

PiperOrigin-RevId: 895495564

objectivec/GPBUtilities.m

@@ -1987,7 +1987,7 @@ static void AppendTextFormatForUnknownFields(GPBUnknownFields *ufs, NSMutableStr
       } break;
     }
   }
-  [subIndent release];
+
   [sortedFields release];
 }
 

objectivec/Tests/GPBUtilitiesTests.m

@@ -214,6 +214,29 @@
   XCTAssertEqualObjects(expected, result);
 }
 
+- (void)testTextFormatUnknownFieldsCrash {
+  GPBUnknownFields *ufs = [[[GPBUnknownFields alloc] init] autorelease];
+  GPBUnknownFields *current = ufs;
+
+  // Nest single groups deep enough to force a heap-allocated indent string.
+  for (int i = 0; i < 15; ++i) {
+    current = [current addGroupWithFieldNumber:150];
+  }
+
+  // Now add multiple siblings at that deep level to trigger reuse/release.
+  for (int i = 0; i < 10; ++i) {
+    GPBUnknownFields *sibling = [current addGroupWithFieldNumber:151];
+    [sibling addFieldNumber:1 varint:1];
+  }
+
+  TestEmptyMessage *message = [TestEmptyMessage message];
+  XCTAssertTrue([message mergeUnknownFields:ufs extensionRegistry:nil error:NULL]);
+
+  @autoreleasepool {
+    [message description];
+  }
+}
+
 - (void)testSetRepeatedFields {
   TestAllTypes *message = [TestAllTypes message];