[BitcodeReader] Add bounds checking on Strtab (#76403)

This is needed to protect against global overflows, which was found by a
fuzzer recently.

Fixes: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=65283

---------

Signed-off-by: David Korczynski <[email protected]>

Author: DavidKorczynski

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -4218,6 +4218,9 @@ Error BitcodeReader::parseGlobalIndirectSymbolRecord(
 
   // Check whether we have enough values to read a partition name.
   if (OpNum + 1 < Record.size()) {
+    // Check Strtab has enough values for the partition.
+    if (Record[OpNum] + Record[OpNum + 1] > Strtab.size())
+      return error("Malformed partition, too large.");
     NewGA->setPartition(
         StringRef(Strtab.data() + Record[OpNum], Record[OpNum + 1]));
     OpNum += 2;