Merge bitcoin-core/secp256k1#1442: Return temporaries to being unsigned in secp256k1_fe_sqr_inner

1027135 Return temporaries to being unsigned in secp256k1_fe_sqr_inner (roconnor-blockstream)

Pull request description:

  These temporaries seem to been inadvertently changed to signed during a refactoring.  Generally, bit shifting is frowned upon for signed values.

ACKs for top commit:
  sipa:
    utACK 1027135
  real-or-random:
    utACK 1027135

Tree-SHA512: a9fefe4b146163209662cd435422beb3c9561eb9e83110454184f70df2292992f39ec1971143428e039a80cad2f6285db74de2f059e877ad8756ff739269b67a

Author: real-or-random

src/field_5x52_int128_impl.h

@@ -159,7 +159,7 @@ SECP256K1_INLINE static void secp256k1_fe_mul_inner(uint64_t *r, const uint64_t
 SECP256K1_INLINE static void secp256k1_fe_sqr_inner(uint64_t *r, const uint64_t *a) {
     secp256k1_uint128 c, d;
     uint64_t a0 = a[0], a1 = a[1], a2 = a[2], a3 = a[3], a4 = a[4];
-    int64_t t3, t4, tx, u0;
+    uint64_t t3, t4, tx, u0;
     const uint64_t M = 0xFFFFFFFFFFFFFULL, R = 0x1000003D10ULL;
 
     VERIFY_BITS(a[0], 56);