efi-string: Fix strchr() null byte handling

strchr() should be able to search for the terminating null byte,
our implementation doesn't, let's fix that.

src/boot/efi/efi-string.c

@@ -112,7 +112,7 @@ DEFINE_STRCPY(char16_t, strcpy16);
                         s++;                       \
                 }                                  \
                                                    \
-                return NULL;                       \
+                return c ? NULL : (type *) s;      \
         }
 
 DEFINE_STRCHR(char, strchr8);

src/boot/efi/test-efi-string.c

@@ -229,6 +229,8 @@ TEST(strchr8) {
         assert_se(strchr8(str, 'a') == &str[0]);
         assert_se(strchr8(str, 'c') == &str[2]);
         assert_se(strchr8(str, 'B') == &str[4]);
+
+        assert_se(strchr8(str, 0) == str + strlen8(str));
 }
 
 TEST(strchr16) {
@@ -240,6 +242,8 @@ TEST(strchr16) {
         assert_se(strchr16(str, 'a') == &str[0]);
         assert_se(strchr16(str, 'c') == &str[2]);
         assert_se(strchr16(str, 'B') == &str[4]);
+
+        assert_se(strchr16(str, 0) == str + strlen16(str));
 }
 
 TEST(xstrndup8) {