debug/dwarf: better error message when reading absent debug_line_str

When a DW_FORM_line_strp is used without a debug_line_str section a
good error message (about the missing section) is generated but
immediately overwritten by the underflow error generated by trying to
read the non-existent section.

Updates golang#49590

Change-Id: I1c431392123a86c78c95ef1f185ebd6f17f2476a
Reviewed-on: https://go-review.googlesource.com/c/go/+/363894
Run-TryBot: Alessandro Arzilli <alessandro.arzilli@gmail.com>
Trust: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/debug/dwarf/entry.go

@@ -641,6 +641,7 @@ func (b *buf) entry(cu *Entry, atab abbrevTable, ubase Offset, vers int) *Entry
 			} else {
 				if len(b.dwarf.lineStr) == 0 {
 					b.error("DW_FORM_line_strp with no .debug_line_str section")
+					return nil
 				}
 				b1 = makeBuf(b.dwarf, b.format, "line_str", 0, b.dwarf.lineStr)
 			}