[analyzer] Fix a crash in SyntaxCString checker when given a custom strncat.

Fixes PR13476

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179228 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/StaticAnalyzer/Checkers/CStringSyntaxChecker.cpp b/lib/StaticAnalyzer/Checkers/CStringSyntaxChecker.cpp
index 3a57a56..92c0eef 100644
--- a/lib/StaticAnalyzer/Checkers/CStringSyntaxChecker.cpp
+++ b/lib/StaticAnalyzer/Checkers/CStringSyntaxChecker.cpp
@@ -101,6 +101,8 @@
 //   - strncat(dst, src, sizeof(dst) - 1);
 //   - strncat(dst, src, sizeof(dst));
 bool WalkAST::containsBadStrncatPattern(const CallExpr *CE) {
+  if (CE->getNumArgs() != 3)
+    return false;
   const Expr *DstArg = CE->getArg(0);
   const Expr *SrcArg = CE->getArg(1);
   const Expr *LenArg = CE->getArg(2);