btf: fix out-of-bounds panic

debug/elf doesn't seem to validate that section numbers are valid,
so we can trigger a crash here. Unfortunately I wasn't able to minimize
the crasher, so this change doesn't have an accompanying test.

Found using go-fuzz.

internal/btf/btf.go

@@ -80,6 +80,10 @@ func LoadSpecFromReader(rd io.ReaderAt) (*Spec, error) {
 			continue
 		}
 
+		if int(symbol.Section) >= len(file.Sections) {
+			return nil, fmt.Errorf("symbol %s: invalid section %d", symbol.Name, symbol.Section)
+		}
+
 		secName := file.Sections[symbol.Section].Name
 		if _, ok := sectionSizes[secName]; !ok {
 			continue