compile-loc2c: Fix uninitialized variable error

Compiling with clang gives this warning/error:

  /home/emaisin/src/binutils-gdb/gdb/compile/compile-loc2c.c:731:6: error: variable 'uoffset' is uninitialized when used here [-Werror,-Wuninitialized]
              uoffset += dwarf2_per_cu_text_offset (per_cu);
              ^~~~~~~
  /home/emaisin/src/binutils-gdb/gdb/compile/compile-loc2c.c:669:23: note: initialize the variable 'uoffset' to silence this warning
        uint64_t uoffset, reg;
                        ^
                         = 0

I am really not sure if what this patch does is good, but it is my best
guess.  DW_OP_addr means that there's an constant address provided by
the DWARF bytecode that should be pushed on the stack.  That address is
considered skipped by the "op_ptr += addr_size", but it is never read.
uoffset is indeed read just after, without having been assigned first.

So I think the intent is to read the address, it was just omitted.

gdb/ChangeLog:

	* compile/compile-loc2c.c (do_compile_dwarf_expr_to_c): Read
	address when op is DW_OP_addr.

gdb/compile/compile-loc2c.c

@@ -722,6 +722,7 @@ do_compile_dwarf_expr_to_c (int indent, string_file &stream,
 	  break;
 
 	case DW_OP_addr:
+	  uoffset = extract_unsigned_integer (op_ptr, addr_size, byte_order);
 	  op_ptr += addr_size;
 	  /* Some versions of GCC emit DW_OP_addr before
 	     DW_OP_GNU_push_tls_address.  In this case the value is an