rv_histogram: Make it more verbose

Section headers must be present for static analysis

Closed #272

tools/rv_histogram.c

@@ -231,21 +231,32 @@ static void insn_hist_incr(const rv_insn_t *ir)
 
 int main(int argc, const char *args[])
 {
+    int ret_val = 0;
+
     if (!parse_args(argc, args) || !elf_prog) {
         print_usage(args[0]);
-        return 1;
+        ret_val = 1;
+        goto end;
     }
 
     /* resolver of histogram accounting */
     hist_record_handler hist_record = show_reg ? reg_hist_incr : insn_hist_incr;
 
     elf_t *e = elf_new();
     if (!elf_open(e, elf_prog)) {
-        fprintf(stderr, "elf_open failed\n");
-        return 1;
+        (void) fprintf(stderr, "elf_open %s failed\n", elf_prog);
+        ret_val = 1;
+        goto end;
     }
 
     struct Elf32_Ehdr *hdr = get_elf_header(e);
+    if (!hdr->e_shnum) {
+        (void) fprintf(stderr, "no section headers are found in %s\n",
+                       elf_prog);
+        ret_val = 1;
+        goto end;
+    }
+
     uint8_t *elf_first_byte = get_elf_first_byte(e);
     const struct Elf32_Shdr **shdrs =
         (const struct Elf32_Shdr **) &elf_first_byte[hdr->e_shoff];
@@ -317,5 +328,6 @@ int main(int argc, const char *args[])
 
     elf_delete(e);
 
-    return 0;
+end:
+    return !!ret_val;
 }