debug/buildinfo: don't crash on corrupt object file

If the length reported for the object file is more than the amount of
data we actually read, then the count can tell us that there is
sufficient remaining data but the slice operation can fail.

No test case because the problem can only happen for invalid data.
Let the fuzzer find cases like this.

Fixes #69066

Change-Id: I8d12ca8ade3330517ade45c7578b477772b7efd2
Reviewed-on: https://go-review.googlesource.com/c/go/+/608517
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/debug/buildinfo/buildinfo.go

@@ -380,7 +380,14 @@ func searchMagic(x exe, start, size uint64) (uint64, error) {
 			}
 			if i%buildInfoAlign != 0 {
 				// Found magic, but misaligned. Keep searching.
-				data = data[(i+buildInfoAlign-1)&^(buildInfoAlign-1):]
+				next := (i + buildInfoAlign - 1) &^ (buildInfoAlign - 1)
+				if next > len(data) {
+					// Corrupt object file: the remaining
+					// count says there is more data,
+					// but we didn't read it.
+					return 0, errNotGoExe
+				}
+				data = data[next:]
 				continue
 			}
 			// Good match!