rv_histogram: Make it more verbose

section headers must be present for static analysis

Closed #272

tools/rv_histogram.c

@@ -241,11 +241,16 @@ int main(int argc, const char *args[])
 
     elf_t *e = elf_new();
     if (!elf_open(e, elf_prog)) {
-        fprintf(stderr, "elf_open failed\n");
+        fprintf(stderr, "elf_open %s failed\n", elf_prog);
         return 1;
     }
 
     struct Elf32_Ehdr *hdr = get_elf_header(e);
+    if(!hdr->e_shnum){
+        fprintf(stderr, "no section headers are found in %s\n", elf_prog);
+        return 1;
+    }
+
     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];