Complain if an elf file contains no debug info

If an elf file was stripped of debug info, then a2ltool would previously
still accept it, and remove all data from the a2l file during an update.
Now an elf file without debug info is rejected.

src/dwarf/mod.rs

@@ -109,8 +109,17 @@ impl DebugData {
     pub(crate) fn load(filename: &OsStr, verbose: bool) -> Result<Self, String> {
         let filedata = load_filedata(filename)?;
         let elffile = load_elf_file(&filename.to_string_lossy(), &filedata)?;
+
+        if elffile.sections().find(|section| section.name() == Ok(".debug_info")).is_none() {
+            return Err(format!("Error: {} does not contain DWARF2+ debug info. The section .debug_info is missing.", filename.to_string_lossy()));
+        }
+
         let dwarf = load_dwarf(&elffile)?;
 
+        if !verify_dwarf_compile_units(&dwarf) {
+            return Err(format!("Error: {} does not contain DWARF2+ debug info - zero compile units contain debug info.", filename.to_string_lossy()));
+        }
+
         let sections = get_elf_sections(&elffile);
 
         let dbg_reader = DebugDataReader {
@@ -187,6 +196,17 @@ fn load_dwarf<'data>(
     gimli::Dwarf::load(loader)
 }
 
+// verify that the dwarf data is valid
+fn verify_dwarf_compile_units(dwarf: &gimli::Dwarf<SliceType>) -> bool {
+    let mut units_iter = dwarf.debug_info.units();
+    let mut units_count = 0;
+    while let Ok(Some(_)) = units_iter.next() {
+        units_count += 1;
+    }
+
+    units_count > 0
+}
+
 // get a section from the elf file.
 // returns a slice referencing the section data if it exists, or an empty slice otherwise
 fn get_file_section_reader<'data>(