[BinaryFormat] Fix out of bounds read.

Found by OSS-FUZZ!
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3220

llvm-svn: 312238

llvm/lib/BinaryFormat/Magic.cpp

@@ -182,7 +182,7 @@ file_magic llvm::identify_magic(StringRef Magic) {
     break;
 
   case 'M': // Possible MS-DOS stub on Windows PE file
-    if (startswith(Magic, "MZ")) {
+    if (startswith(Magic, "MZ") && Magic.size() >= 0x3c + 4) {
       uint32_t off = read32le(Magic.data() + 0x3c);
       // PE/COFF file, either EXE or DLL.
       if (off < Magic.size() &&