builtin: add sanity check on the number of returned frames by C.backtrace

vlib/builtin/builtin_nix.c.v

@@ -50,6 +50,10 @@ fn print_backtrace_skipping_top_frames_mac(skipframes int) bool {
 	$if macos {
 		buffer := [100]byteptr
 		nr_ptrs := C.backtrace(buffer, 100)
+		if nr_ptrs < 2 {
+			eprintln('C.backtrace returned less than 2 frames')
+			return false
+		}
 		C.backtrace_symbols_fd(&buffer[skipframes], nr_ptrs - skipframes, 2)
 	}
 	return true
@@ -59,6 +63,10 @@ fn print_backtrace_skipping_top_frames_freebsd(skipframes int) bool {
 	$if freebsd {
 		buffer := [100]byteptr
 		nr_ptrs := C.backtrace(buffer, 100)
+		if nr_ptrs < 2 {
+			eprintln('C.backtrace returned less than 2 frames')
+			return false
+		}
 		C.backtrace_symbols_fd(&buffer[skipframes], nr_ptrs - skipframes, 2)
 	}
 	return true
@@ -81,6 +89,10 @@ fn print_backtrace_skipping_top_frames_linux(skipframes int) bool {
 	}
 	buffer := [100]byteptr
 	nr_ptrs := C.backtrace(buffer, 100)
+	if nr_ptrs < 2 {
+		eprintln('C.backtrace returned less than 2 frames')
+		return false
+	}
 	nr_actual_frames := nr_ptrs - skipframes
 	mut sframes := []string{}
 	//////csymbols := backtrace_symbols(*voidptr(&buffer[skipframes]), nr_actual_frames)

vlib/builtin/builtin_windows.c.v

@@ -103,10 +103,14 @@ $if msvc {
 	syminitok := C.SymInitialize( handle, 0, 1)
 	if syminitok != 1 {
 		eprintln('Failed getting process: Aborting backtrace.\n')
-		return true
+		return false
 	}
 
 	frames := int(C.CaptureStackBackTrace(skipframes + 1, 100, backtraces, 0))
+	if frames < 2 {
+		eprintln('C.CaptureStackBackTrace returned less than 2 frames')
+		return false
+	}
 	for i in 0..frames {
 		frame_addr := backtraces[i]
 		if C.SymFromAddr(handle, frame_addr, &offset, si) == 1 {