Fix fft.cpp uninitialized mem warnings found in Ubuntu

Here's the warning:
fft.cpp: In function ‘bool Fft_transformBluestein(double*, double*, size_t)’:
fft.cpp:163:31: warning: ‘*cos_table’ may be used uninitialized [-Wmaybe-uninitialized]
  163 |         breal[0] = cos_table[0];
      |                    ~~~~~~~~~~~^
fft.cpp:164:31: warning: ‘*sin_table’ may be used uninitialized [-Wmaybe-uninitialized]
  164 |         bimag[0] = sin_table[0];
      |                    ~~~~~~~~~~~^

Just needed to return if the malloc size is 0.

libkoviz/fft.cpp

@@ -127,6 +127,9 @@ bool Fft_transformBluestein(double real[], double imag[], size_t n) {
 	}
 
 	// Allocate memory
+	if ( n <= 0 || m <= 0 ) {
+            return false;
+        }
     if (FFT_SIZE_MAX / sizeof(double) < n || FFT_SIZE_MAX / sizeof(double) < m)
 		return false;
 	size_t size_n = n * sizeof(double);