runtime: only clear key string's pointer in mapdelete_faststr

Change-Id: I0360d294868ec4423e4ae036009fac4e72425c9c
Reviewed-on: https://go-review.googlesource.com/59152
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/hashmap_fast.go

@@ -754,7 +754,8 @@ search:
 			if k.str != key.str && !memequal(k.str, key.str, uintptr(key.len)) {
 				continue
 			}
-			*(*string)(kptr) = ""
+			// Clear key's pointer.
+			k.str = nil
 			// Only clear value if there are pointers in it.
 			if t.elem.kind&kindNoPointers == 0 {
 				v := add(unsafe.Pointer(b), dataOffset+bucketCnt*2*sys.PtrSize+i*uintptr(t.valuesize))