dwarf/reader: precalcStack does not need to read past the first entry

It was reading all the way to the end of the debug_info section,
slowing down stacktraces substantially.

Benchmark before:

BenchmarkConditionalBreakpoints-4   	       1	80344642562 ns/op

Benchmark after:

BenchmarkConditionalBreakpoints-4   	       1	22218288218 ns/op

i.e. a reduction of the cost of a breakpoint hit from 8ms to 2.2ms

Updates #1549

pkg/dwarf/reader/reader.go

@@ -426,6 +426,9 @@ childLoop:
 		default:
 			irdr.reader.SkipChildren()
 		}
+		if rentry == nil {
+			break
+		}
 	}
 
 	if rentry != nil && rentry.Tag == dwarf.TagInlinedSubroutine {