Generate assertions in statuslookupgen to validate input

Although inputs to this function have to come from an enum that is
generated by the same X-macro that is used to build the perfect hash
table, anything could theoretically be passed to it if casted from an
integer.  So assert that the returned string actually matches the
requested HTTP status code.

(The lookup function wouldn't crash otherwise, but could return either
a 999 Invalid code, or some code that is valid but has nothing to do
with the requested code.  Since this is just an assertion, this is
mostly here to ensure that issues found during debugging are actually
caught.)

src/bin/tools/statuslookupgen.c

@@ -92,7 +92,11 @@ int main(void)
 
     printf("\n");
     printf("    const uint32_t k = (uint32_t)status - %d;\n", best_subtract);
-    printf("    return table[((k << %d) | (k >> %d)) %% %d];\n", 32 - best_rot, best_rot, best_mod);
+    printf("    const char *ret = table[((k << %d) | (k >> %d)) %% %d];\n", 32 - best_rot, best_rot, best_mod);
+    printf("    assert((uint32_t)(ret[2] - '0') == ((uint32_t)status %% 10));\n");
+    printf("    assert((uint32_t)(ret[1] - '0') == ((uint32_t)(status / 10) %% 10));\n");
+    printf("    assert((uint32_t)(ret[0] - '0') == ((uint32_t)(status / 100) %% 10));\n");
+    printf("    return ret;\n");
 
     printf("}\n");