RISC-V: Improve handling of mapping symbols with dot suffix

This commit makes minor improvements to mapping symbols (executable)
handling with a dot suffix.

1.  Use size_t instead of int
2.  Allocate minimum size for the architectural string buffer.
3.  memcpy instead of strncpy because we know the exact size to copy.
4.  Minor variable naming changes.

opcodes/ChangeLog:

	* riscv-dis.c (riscv_get_map_state): Minor improvements to
	handling of executable mapping symbols with dot suffix.

opcodes/riscv-dis.c

@@ -875,12 +875,12 @@ riscv_get_map_state (int n,
       char *suffix = strchr (name, '.');
       if (suffix)
 	{
-	  int suffix_index = (int)(suffix - name);
-	  char *name_substr = xmalloc (suffix_index + 1);
-	  strncpy (name_substr, name, suffix_index);
-	  name_substr[suffix_index] = '\0';
-	  riscv_parse_subset (&riscv_rps_dis, name_substr + 2);
-	  free (name_substr);
+	  size_t arch_len = (size_t) (suffix - name) - 2;
+	  char *arch = xmalloc (arch_len + 1);
+	  memcpy (arch, name + 2, arch_len);
+	  arch[arch_len] = '\0';
+	  riscv_parse_subset (&riscv_rps_dis, arch);
+	  free (arch);
 	}
       else
 	riscv_parse_subset (&riscv_rps_dis, name + 2);