debug/dwarf: formStrp uses a 64-bit value for 64-bit DWARF

No test as the only system I know that uses 64-bit DWARF is AIX.

Change-Id: I24e225253075be188845656b6778993c2d24ebf5
Reviewed-on: https://go-review.googlesource.com/84379
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/debug/dwarf/entry.go

@@ -461,7 +461,18 @@ func (b *buf) entry(atab abbrevTable, ubase Offset) *Entry {
 		case formString:
 			val = b.string()
 		case formStrp:
-			off := b.uint32() // offset into .debug_str
+			var off uint64 // offset into .debug_str
+			is64, known := b.format.dwarf64()
+			if !known {
+				b.error("unknown size for DW_FORM_strp")
+			} else if is64 {
+				off = b.uint64()
+			} else {
+				off = uint64(b.uint32())
+			}
+			if uint64(int(off)) != off {
+				b.error("DW_FORM_strp offset out of range")
+			}
 			if b.err != nil {
 				return nil
 			}