[CodeCompletion] Fix a crash when completing typealiases for protocol conformance (#3120)

The index may be at the end of the ArrayRef of chunks if the completion
ends with a simple parameter with no type annotation.  Check that the
index is in-bounds before adding text.

rdar://problem/26273906

Author: benlangmuir

test/SourceKit/CodeComplete/complete_structure.swift

@@ -7,6 +7,7 @@
 // RUN: %complete-test %s -group=none -fuzz -structure -tok=OVERRIDE_0 | FileCheck %s -check-prefix=OVERRIDE_0
 // RUN: %complete-test %s -group=none -fuzz -structure -tok=S1_INNER_0 | FileCheck %s -check-prefix=S1_INNER_0
 // RUN: %complete-test %s -group=none -fuzz -structure -tok=INT_INNER_0 | FileCheck %s -check-prefix=INT_INNER_0
+// RUN: %complete-test %s -group=none -fuzz -structure -tok=ASSOCIATED_TYPE_1 | FileCheck %s -check-prefix=ASSOCIATED_TYPE_1
 
 struct S1 {
   func method1() {}
@@ -119,3 +120,11 @@ func test9(_ x: inout Int) {
 // INT_INNER_0: {name:x++}
 // INT_INNER_0: {name:x>>}
 // INT_INNER_0: {name:x..<}
+
+protocol P1 {
+  associatedtype T
+}
+struct S2: P1 {
+  #^ASSOCIATED_TYPE_1^#
+}
+// ASSOCIATED_TYPE_1: {name:T = }{params:{l:Type}}

tools/SourceKit/lib/SwiftLang/SwiftCompletion.cpp

@@ -372,7 +372,7 @@ static void getResultStructure(
       if (!param.range().empty())
         parameters.push_back(std::move(param));
 
-      if (chunks[i].hasText())
+      if (i < chunks.size() && chunks[i].hasText())
         textSize += chunks[i].getText().size();
     }