[llvm-readobj][XCOFF] Warn about invalid offset

Followup for D105522

Differential Revision: https://reviews.llvm.org/D107398

Author: vitalybuka

llvm/test/tools/llvm-readobj/XCOFF/string-table.yaml

@@ -32,9 +32,10 @@ Symbols:
 
 ## There is no string table.
 # RUN: yaml2obj --docnum=3 %s -o %t3
-# RUN: llvm-readobj --string-table %t3 | FileCheck %s --check-prefix=NO-STRTBL
+# RUN: llvm-readobj --string-table %t3 2>&1 | FileCheck %s --check-prefix=NO-STRTBL
 
 # NO-STRTBL:      StringTable {
+# NO-STRTBL-NEXT: error: offset is out of string contents
 # NO-STRTBL-NEXT: }
 
 --- !XCOFF

llvm/tools/llvm-readobj/ObjDumper.cpp

@@ -54,8 +54,10 @@ static void printAsPrintable(raw_ostream &W, const uint8_t *Start, size_t Len) {
 
 void ObjDumper::printAsStringList(StringRef StringContent,
                                   size_t StringDataOffset) {
-  if (StringContent.size() < StringDataOffset)
+  if (StringContent.size() < StringDataOffset) {
+    reportUniqueWarning("error: offset is out of string contents");
     return;
+  }
   const uint8_t *StrContent = StringContent.bytes_begin();
   // Some formats contain additional metadata at the start which should not be
   // interpreted as strings. Skip these bytes, but account for them in the