gopack: fix prefix bug

When prefix doesn't match, was deleting entire .6 file.

R=dsymonds
CC=golang-dev
https://golang.org/cl/4427041

src/cmd/gopack/ar.c

@@ -1676,6 +1676,10 @@ arread_cutprefix(Biobuf *b, Armember *bp)
 					offset = o;
 				}
 			}
+		} else {
+			// didn't find the whole prefix.
+			// give up and let it emit the entire name.
+			inprefix = nil;
 		}
 
 		// copy instructions