grep: fix echo "aa" | busybox grep -F -w "a" (should not match)

function                                             old     new   delta
grep_file                                           1461    1470      +9

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/grep.tests

@@ -165,6 +165,12 @@ testing "grep -w word doesn't match wordword" \
 	"wordword\n" \
 	""
 
+testing "grep -F -w w doesn't match ww" \
+	"grep -F -w w input" \
+	"" \
+	"ww\n" \
+	""
+
 testing "grep -w word match second word" \
 	"grep -w word input" \
 	"bword,word\n""wordb,word\n""bwordb,word\n" \