Don't assert if materializing before seeing any function bodies

This assert was reachable from user input. A minimized test case (no
FUNCTION_BLOCK_ID record) is attached.

Bug found with afl-fuzz

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251910 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Reader/BitcodeReader.cpp

@@ -3055,7 +3055,9 @@ std::error_code BitcodeReader::rememberAndSkipFunctionBodies() {
   if (Stream.AtEndOfStream())
     return error("Could not find function in stream");
 
-  assert(SeenFirstFunctionBody);
+  if (!SeenFirstFunctionBody)
+    return error("Trying to materialize functions before seeing function blocks");
+
   // An old bitcode file with the symbol table at the end would have
   // finished the parse greedily.
   assert(SeenValueSymbolTable);