debug/elf: use saferio.SliceCap when decoding ELF sections

This avoids allocating an overly large slice for corrupt input.

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

src/debug/elf/file.go

@@ -467,8 +467,12 @@ func NewFile(r io.ReaderAt) (*File, error) {
 	}
 
 	// Read section headers
-	f.Sections = make([]*Section, shnum)
-	names := make([]uint32, shnum)
+	c := saferio.SliceCap((*Section)(nil), uint64(shnum))
+	if c < 0 {
+		return nil, &FormatError{0, "too many sections", shnum}
+	}
+	f.Sections = make([]*Section, 0, c)
+	names := make([]uint32, 0, c)
 	for i := 0; i < shnum; i++ {
 		off := shoff + int64(i)*int64(shentsize)
 		sr.Seek(off, seekStart)
@@ -479,7 +483,7 @@ func NewFile(r io.ReaderAt) (*File, error) {
 			if err := binary.Read(sr, f.ByteOrder, sh); err != nil {
 				return nil, err
 			}
-			names[i] = sh.Name
+			names = append(names, sh.Name)
 			s.SectionHeader = SectionHeader{
 				Type:      SectionType(sh.Type),
 				Flags:     SectionFlag(sh.Flags),
@@ -496,7 +500,7 @@ func NewFile(r io.ReaderAt) (*File, error) {
 			if err := binary.Read(sr, f.ByteOrder, sh); err != nil {
 				return nil, err
 			}
-			names[i] = sh.Name
+			names = append(names, sh.Name)
 			s.SectionHeader = SectionHeader{
 				Type:      SectionType(sh.Type),
 				Flags:     SectionFlag(sh.Flags),
@@ -544,7 +548,7 @@ func NewFile(r io.ReaderAt) (*File, error) {
 			}
 		}
 
-		f.Sections[i] = s
+		f.Sections = append(f.Sections, s)
 	}
 
 	if len(f.Sections) == 0 {