sh: Don't lose $? when backquoted command ends with semicolon or newline.

An empty simple command was added and overwrote the exit status with 0.

This affects `...` but not $(...).

Example:
  v=`false;`; echo $?

bin/sh/parser.c

@@ -286,7 +286,8 @@ list(int nlflag, int erflag)
 				tokpushback++;
 			}
 			checkkwd = CHKNL | CHKKWD | CHKALIAS;
-			if (!nlflag && !erflag && tokendlist[peektoken()])
+			if (!nlflag && (erflag ? peektoken() == TEOF :
+			    tokendlist[peektoken()]))
 				return ntop;
 			break;
 		case TEOF:

tools/regression/bin/sh/expansion/cmdsubst14.0

@@ -0,0 +1,5 @@
+# $FreeBSD$
+
+! v=`false
+
+`

tools/regression/bin/sh/expansion/cmdsubst15.0

@@ -0,0 +1,5 @@
+# $FreeBSD$
+
+! v=`false;
+
+`