Silence some warnings from pedantic static analysis tools, improve compatibility with C++.

C doesn't include the null in an array initilized from a
string literal if it doesn't fit, in C++ this is invalid.

The vararray style prototypes and init+calc also changed in
 this commit are not C89 enough for some tools.

src/ecmult_gen_impl.h

@@ -46,7 +46,7 @@ static void secp256k1_ecmult_gen_start(void) {
 
     /* Construct a group element with no known corresponding scalar (nothing up my sleeve). */
     {
-        static const unsigned char nums_b32[32] = "The scalar for this x is unknown";
+        static const unsigned char nums_b32[33] = "The scalar for this x is unknown";
         secp256k1_fe_t nums_x;
         secp256k1_ge_t nums_ge;
         VERIFY_CHECK(secp256k1_fe_set_b32(&nums_x, nums_b32));

src/field_10x26_impl.h

@@ -1064,7 +1064,9 @@ static void secp256k1_fe_sqr(secp256k1_fe_t *r, const secp256k1_fe_t *a) {
 }
 
 static SECP256K1_INLINE void secp256k1_fe_storage_cmov(secp256k1_fe_storage_t *r, const secp256k1_fe_storage_t *a, int flag) {
-    uint32_t mask0 = flag + ~((uint32_t)0), mask1 = ~mask0;
+    uint32_t mask0, mask1;
+    mask0 = flag + ~((uint32_t)0);
+    mask1 = ~mask0;
     r->n[0] = (r->n[0] & mask0) | (a->n[0] & mask1);
     r->n[1] = (r->n[1] & mask0) | (a->n[1] & mask1);
     r->n[2] = (r->n[2] & mask0) | (a->n[2] & mask1);

src/field_5x52_impl.h

@@ -400,7 +400,9 @@ static void secp256k1_fe_sqr(secp256k1_fe_t *r, const secp256k1_fe_t *a) {
 }
 
 static SECP256K1_INLINE void secp256k1_fe_storage_cmov(secp256k1_fe_storage_t *r, const secp256k1_fe_storage_t *a, int flag) {
-    uint64_t mask0 = flag + ~((uint64_t)0), mask1 = ~mask0;
+    uint64_t mask0, mask1;
+    mask0 = flag + ~((uint64_t)0);
+    mask1 = ~mask0;
     r->n[0] = (r->n[0] & mask0) | (a->n[0] & mask1);
     r->n[1] = (r->n[1] & mask0) | (a->n[1] & mask1);
     r->n[2] = (r->n[2] & mask0) | (a->n[2] & mask1);

src/scalar_8x32_impl.h

@@ -462,7 +462,7 @@ static void secp256k1_scalar_reduce_512(secp256k1_scalar_t *r, const uint32_t *l
     secp256k1_scalar_reduce(r, c + secp256k1_scalar_check_overflow(r));
 }
 
-static void secp256k1_scalar_mul_512(uint32_t l[16], const secp256k1_scalar_t *a, const secp256k1_scalar_t *b) {
+static void secp256k1_scalar_mul_512(uint32_t *l, const secp256k1_scalar_t *a, const secp256k1_scalar_t *b) {
     /* 96 bit accumulator. */
     uint32_t c0 = 0, c1 = 0, c2 = 0;
 
@@ -550,7 +550,7 @@ static void secp256k1_scalar_mul_512(uint32_t l[16], const secp256k1_scalar_t *a
     l[15] = c0;
 }
 
-static void secp256k1_scalar_sqr_512(uint32_t l[16], const secp256k1_scalar_t *a) {
+static void secp256k1_scalar_sqr_512(uint32_t *l, const secp256k1_scalar_t *a) {
     /* 96 bit accumulator. */
     uint32_t c0 = 0, c1 = 0, c2 = 0;