[libc] Fix issue with fuzz input too short for atoi diff fuzz

The string to integer differential fuzzer assumes at least one byte of
meaningful input, but wasn't explicitly checking that. Now it does.

Author: michaelrj-google

libc/fuzzing/stdlib/strtointeger_differential_fuzz.cpp

@@ -44,6 +44,10 @@
 // greater than 50% chance for each character to end the string, making the odds
 // of getting long numbers very low.
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  if (size < 2) // Needs at least one byte for the base and one byte for the
+                // string.
+    return 0;
+
   uint8_t *container = new uint8_t[size + 1];
   if (!container)
     __builtin_trap();