build: warn on potentially uninitialized reads

Enable -Wconditional-uninitialized to warn on potentially uninitialized
reads.

Fix the sole such warning in Bitcoin Core in GetRdRand(): r1 would be
set to 0 on rdrand failure, so initializing it to 0 is a non-functional
change.

From "Intel 64 and IA-32 ArchitecturesSoftware Developer's Manual" [1],
page 1711: "CF=1 indicates that the data in the destination is valid.
Otherwise CF=0 and the data in the destination operand will be returned
as zeros for the specified width."

[1] https://software.intel.com/sites/default/files/managed/39/c5/325462-sdm-vol-1-2abcd-3abcd.pdf

src/random.cpp

@@ -116,7 +116,10 @@ static uint64_t GetRdRand() noexcept
     // RdRand may very rarely fail. Invoke it up to 10 times in a loop to reduce this risk.
 #ifdef __i386__
     uint8_t ok;
-    uint32_t r1, r2;
+    // Initialize to 0 to silence a compiler warning that r1 or r2 may be used
+    // uninitialized. Even if rdrand fails (!ok) it will set the output to 0,
+    // but there is no way that the compiler could know that.
+    uint32_t r1 = 0, r2 = 0;
     for (int i = 0; i < 10; ++i) {
         __asm__ volatile (".byte 0x0f, 0xc7, 0xf0; setc %1" : "=a"(r1), "=q"(ok) :: "cc"); // rdrand %eax
         if (ok) break;
@@ -128,7 +131,7 @@ static uint64_t GetRdRand() noexcept
     return (((uint64_t)r2) << 32) | r1;
 #elif defined(__x86_64__) || defined(__amd64__)
     uint8_t ok;
-    uint64_t r1;
+    uint64_t r1 = 0; // See above why we initialize to 0.
     for (int i = 0; i < 10; ++i) {
         __asm__ volatile (".byte 0x48, 0x0f, 0xc7, 0xf0; setc %1" : "=a"(r1), "=q"(ok) :: "cc"); // rdrand %rax
         if (ok) break;