Brace all the if/for/while.

Unbraced statements spanning multiple lines has been shown in many
 projects to contribute to the introduction of bugs and a failure
 to catch them in review, especially for maintenance on infrequently
 modified code.

Most, but not all, of the existing practice in the codebase were not
 cases that I would have expected to eventually result in bugs but
 applying it as a rule makes it easier for other people to safely
 contribute.

I'm not aware of any such evidence for the case with the statement
 on a single line, but some people strongly prefer to never do that
 and the opposite rule of "_always_ use a single line for single
 statement blocks" isn't a reasonable rule for formatting reasons.
 Might as well brace all these too, since that's more universally
 acceptable.

[In any case, I seem to have introduced the vast majority of the
 single-line form (as they're my preference where they fit).]

This also removes a broken test which is no longer needed.

src/ecmult_gen_impl.h

@@ -35,8 +35,9 @@ static void secp256k1_ecmult_gen_start(void) {
     secp256k1_gej_t nums_gej;
     secp256k1_ecmult_gen_consts_t *ret;
     int i, j;
-    if (secp256k1_ecmult_gen_consts != NULL)
+    if (secp256k1_ecmult_gen_consts != NULL) {
         return;
+    }
 
     /* Allocate the precomputation table. */
     ret = (secp256k1_ecmult_gen_consts_t*)checked_malloc(sizeof(secp256k1_ecmult_gen_consts_t));
@@ -95,8 +96,9 @@ static void secp256k1_ecmult_gen_start(void) {
 
 static void secp256k1_ecmult_gen_stop(void) {
     secp256k1_ecmult_gen_consts_t *c;
-    if (secp256k1_ecmult_gen_consts == NULL)
+    if (secp256k1_ecmult_gen_consts == NULL) {
         return;
+    }
 
     c = (secp256k1_ecmult_gen_consts_t*)secp256k1_ecmult_gen_consts;
     secp256k1_ecmult_gen_consts = NULL;